Capacitively coupled DTMOS on SOI

[0002] The present invention generally relates to the design of field effect transistors (FETS) and, more particularly, to a metal oxide silicon (MOS) transistor structure configured to operate as a dynamic threshold voltage metal oxide silicon (DTMOS) structure which facilitates mitigation of the operational voltage limitation associated with conventional DTMOS transistor structures.

[0003] As is known in the art, transistors such as metal oxide silicon (MOS) transistors, have been formed in isolated regions of a semiconductor body such as an epitaxial layer which was itself formed on a semiconductor, typically bulk silicon, substrate. With an n-channel MOS field effect transistor (FET), the body is of p-type conductivity and the source and drain regions are formed in the p-type conductivity body as N⁺type conductivity regions. With a p-channel MOSFET, the body, or epitaxial layer, is of n-type conductivity and the source and drain regions are formed in the n-type conductivity body as P⁺type conductivity regions. It has been suggested that the semiconductor body, or layer, be formed on an insulating substrate, or over an insulation layer formed in a semiconductor substrate. Such technology sometimes is referred to as Silicon-on-Insulator (SOI) technology. Silicon-on-Insulator MOS technologies have a number of advantages over bulk silicon MOS transistors. These advantages include: reduced source/drain capacitance and hence improved speed performance at higher-operating frequencies; reduced N⁺to P⁺spacing and hence higher packing density due to ease of isolation; and higher “soft error” upset immunity (i.e., the immunity to the effects of alpha particle strikes).

[0004] Silicon-on-Insulator technology is characterized by the formation of a thin silicon layer for formation of the active devices over an insulating layer, such as an oxide, which is in turn formed over a substrate. Transistor sources in drains are formed by, for example, implantations into the silicon layer while transistor gates are formed by forming a patterned oxide and conductor (e.g. metal) layer structure. Such structures provide a significant gain in performance by having lower parasitic capacitance (due to the insulator layer) and increased drain current due to floating body charging effects (since no connection is made to the channel region and charging of the floating body provides access towards a majority of carriers which dynamically lower the threshold voltage, resulting in increased drain current). However, the floating body can introduce dynamic instabilities in the operation of such a transistor.

[0005] An SOI field effect transistor combines two separated immunity groups, generally formed by implantation, constituting the source and drain of the transistor with the general region (device body) between them covered by a thin gate insulator and a conductive gate. Typically no electrical connection is made to the channel region and therefore the body is electrically floating. Because the source and drain regions normally extend entirely through the thin silicon layer, the electrical potential of the body is governed by Kirchoffs current law, wherein the sum of the currents flowing into the body equals the sum of the currents flowing out of the body. Because the channel potential is dependent on the body voltage, the device threshold voltage varies as a function of the body voltage.

[0006] The boundaries between the channel region and the source and drain, respectively, form junctions which are normally reversed biased. Conduction in the channel region normally occurs immediately below the gate insulator in the region in which depletion can be controlled by a gate voltage. However, the junctions at the boundary of the source and drain also form a parasitic lateral bipolar transistor, which, in effect exists somewhat below the field effect transistor and may supplement desired channel current. On the other hand, the parasitic bipolar device cannot be controlled and under some bias conditions, the operation of the parasitic bipolar device may transiently dominate the operation of the field effect transistor and effectively occupy substantially the entire silicon layer at times when the channel current is not desired.

[0007] When the device is switching, the body is coupled to various terminals of the device because there are capacitances between the body and gate, body and source, and body and drain respectively. When the voltage at the various terminal changes, the body voltage changes as a function of time which in turn effects the device threshold voltage. In certain cases, this relationship may be harmful to a device (e.g., inverter). For example, when the gate of an inverter is switched on the drain is discharged (which is typically the output of the inverter)—thus the drain voltage falls when the gate is switched ON. Because the drain and body are capacitively coupled, when the drain voltage drops so does the body voltage. There is an inverse relationship between the body voltage and the threshold voltage. For an NMOS device, when the body voltage falls, the device threshold voltage increases. When the body voltage increases the threshold voltage decreases. Thus, the capacitive coupling between the drain and the body results in the device losing drive current as the device is being switched.

[0008] In SOI transistors there is a lack of a bulk silicon or body contact to the MOS transistor. In some devices, it is desirable to connect the p-type conductivity body in the case of an n-channel MOSFET, or the n-type conductivity body in the case of a p-channel MOSFET, to a fixed potential. This prevents various hysteresis effects associated with having the body potential “float” relative to ground. With bulk silicon MOSFETs such is relatively easy because the bottom of the bulk silicon can be easily electrically connected to a fixed potential.

[0009] SOI devices also exhibit a kink effect which originates from impact ionization. When an SOI MOSFET is operated at a relatively large drain-to-source voltage, channel electrons with sufficient energy cause impact ionization near the drain end of the channel. The generated holes build up in the body of the device, thereby raising the body potential. The increased body potential reduces the threshold voltage of the MOSFET. This increases the MOSFET current and causes the so-called “kink” in SOI MOSFET current vs. voltage (I-V) curves.

[0010] With regard to the lateral bipolar action, if the impact ionization results in a large number of holes, the body bias may be raised sufficiently so that the source region to body p-n junction is forward biased. The resulting emission of minority carriers into the body causes a parasitic npn bipolar transistor between source, body and drain to turn on, leading to loss of gate control over the MOSFET current.

[0011] A solution to controlling floating body effects and threshold voltages is known as a dynamic threshold metal oxide field effect transistor (DTMOS). A large improvement over regular MOSFET can be achieved when the gate and body of the MOSFET are electrically coupled. These devices offer improvements in power consumption in addition to reduced threshold voltages and faster switching times. This advantage is enhanced for SOI devices where base current and capacitances are appreciably reduced because of very small junction areas. However, these device are limited to operation of about a diode drop 0.6-0.8 volts. If the voltage rises above a diode drop, the body to source and body to drain parasitic diodes will turn on and gate control will be lost. This can result in a very high current from source to drain, which may even result in destruction of the device.

[0012] In view of the above, it is apparent that there is an unmet need in the art for a device which mitigates some of the negative effects mentioned above, relating to disadvantages of DTMOS SOI devices.

[0013] The present invention provides for a novel DTMOS device and method for making the same. The device of the present invention mitigates some of the aforementioned problems associated with DTMOS devices. The device of the present invention includes drain and source regions and lightly doped source and drain regions (LDD regions). The device also includes a heavily doped region alongside the drain and source regions and the LDD regions, all of which are disposed under a gate. The heavily doped region provide a capacitive coupling of the gate and body of the DTMOS device. The capacitive coupling combines with the junction capacitance of the device to form a capacitive voltage divider between the drain and body. This provides an ability to operate DTMOS devices above 0.6-0.8 volts resulting in increased switching speeds. Additionally, the device structure mitigates dropping of body potential during device switching by lowering the threshold voltage of the device. As mentioned above, the body potential and threshold potential are related and by controlling body potential, the dropping of body voltage during switching is mitigated. This in turn mitigates variances in the threshold voltage.

[0014] One aspect of the invention relates to a MOSFET device. The device comprises a lightly doped source extension region and a lightly doped drain extension region. The device further includes a heavily doped region residing alongside at least a portion of one of the lightly doped source extension region and the lightly doped drain extension region.

[0015] Another aspect of the invention relates to a transistor device comprising a source region having a N⁺source region and a N⁻lightly doped source region. The structure also comprises a drain region having a N⁺drain region and a N⁻lightly doped drain region. A P⁺⁺heavily doped region is provided. The P⁺⁺region resides alongside a portion of at least one of the N⁻lightly doped source region and the N⁻lightly doped drain region. A P⁺body region resides below a gate of the device and between the source and drain regions. The P⁺⁺heavily doped region provides a capacitive coupling between a body region and the gate of the device and form a capacitive voltage divider with the junction capacitance of the device.

[0016] Yet another aspect of the invention relates to an NMOS device comprising N⁺source and N⁺drain regions formed in a top silicon layer. A P⁺⁺heavily doped region is also formed in the top silicon layer. The P⁺⁺region has a higher dopant concentration than the N⁺source and N⁺drain regions. The P⁺⁺region resides alongside a portion of at least one of the N⁻source and N⁺drain regions. The P⁺⁺region provides a capacitive coupling between a body region and the gate of the device and form a capacitive voltage divider with the junction capacitance of the device.

[0017] Another aspect of the present invention relates to a DTMOS device. The DTMOS device comprises a source region, a drain region, a gate region, a body region and a capacitive system operatively coupled between the gate region and the drain region.

[0018] Still another aspect of the invention relates to a method of forming a MOSFET device. In the method, lightly doped source and drain regions are formed. A heavily doped region is formed and resides alongside at least a portion of at least one of the lightly doped source and drain regions. Source and drain regions are then formed below adjacent the lightly doped regions, respectively. The heavily doped region has a dopant concentration greater than the source and drain regions and the lightly doped regions, and the heavily doped region is implanted at a lower energy level than the source and drain regions and the lightly doped regions.

[0019] Another aspect of the present invention relates to a method of forming an SOI NMOS transistor, comprising the steps of using a SIMOX process to form a silicon base, an oxide layer between the base and a top silicon layer. N⁻lightly doped source and drain extension regions are formed in the top silicon layer. A P⁺⁺heavily doped region is also formed in the top silicon layer. N⁺source and N⁺drain regions are formed in the top silicon layer. The P⁺⁺region has a higher dopant concentration than the N⁺regions. The P⁺⁺regions reside alongside at least a portion of at least one of the N⁻regions. The P⁺⁺regions provide a capacitive coupling between a body region and a gate of the device forming a capacitive voltage divider with the junction capacitance of the device.

[0020] To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.

[0021] FIG. 1ais a schematic cross-sectional side view illustration of a DTMOS SOI structure in accordance with the present invention;

[0022] FIG. 1bis a schematic cross-sectional view illustration of the DTMOS SOI structure of FIG. 1aalong the lines A—A in accordance with the present invention;

[0023] FIG. 1cis a schematic cross-sectional view illustration of the DTMOS SOI structure of FIG. 1aalong the lines B—B in accordance with the present invention;

[0024] FIG. 1dis a schematic illustration of an equivalent circuit of the DTMOS SOI structure of FIGS. 1a-1cin accordance with the present invention;

[0025] FIG. 1eis a graph illustrating gate voltage verse time of the DTMOS SOI structure of FIGS. 1a-1cin accordance with the present invention;

[0026] FIG. 1fis a graph illustrating body voltage verse time of the DTMOS SOI structure of FIGS. 1a-1fcorresponding to the gate voltage illustrated in FIG. 1cin accordance with the present invention;

[0027] FIG. 2 is a schematic cross-sectional illustration of an SOI substrate in accordance with the present invention;

[0028] FIG. 3 is a schematic cross-sectional illustration of the SOI substrate of FIG. 2 with a pad oxide layer and nitride layer formed thereon in accordance with the present invention;

[0029] FIG. 4 is a schematic cross-sectional illustration of the structure of FIG. 3 with isolation regions formed therein in accordance with the present invention;

[0030] FIG. 5 is a schematic cross-sectional illustration of the structure of FIG. 4 with shallow isolation trenches formed at the isolation regions in accordance with the present invention;

[0031] FIG. 6 is a schematic cross-sectional illustration of the structure of FIG. 5 with an oxide layer formed thereon so as to fill the isolation trenches in accordance with the present invention;

[0032] FIG. 7 is a schematic cross-sectional illustration of the structure of FIG. 6 after the oxide layer has been polished down to the surface of the nitride layer in accordance with the present invention;

[0033] FIG. 8 is a schematic cross-sectional illustration of the structure of FIG. 7 after the nitride layer, pad oxide layer and portions of the oxide layer have been etched away in accordance with the present invention;

[0034] FIG. 9 is a schematic cross-sectional illustration of the structure of FIG. 8 undergoing an ion implant step to form a p-type body region in accordance with the present invention;

[0035] FIG. 10 is a schematic cross-sectional illustration of the structure of FIG. 9 undergoing an ion implant step to form a heavily doped region in accordance with the present invention;

[0036] FIG. 11 is a schematic cross-sectional illustration of the structure of FIG. 10 after undergoing the ion implant step to form a heavily doped region in accordance with the present invention;

[0037] FIG. 12 is a schematic cross-sectional illustration of the structure of FIG. 11 with a thin low dielectric constant gate oxide material formed on the substrate surface between the isolation trenches in accordance with the present invention;

[0038] FIG. 13 is a schematic cross-sectional illustration of the structure of FIG. 12 after formation of a gate in accordance with the present invention;

[0039] FIG. 14 is a schematic cross-sectional illustration of the structure of FIG. 13 undergoing an ion implant step to form N⁻source/drain (S/D) lightly doped regions in accordance with the present invention;

[0040] FIG. 15 is a schematic cross-sectional illustration of the structure of FIG. 14 after undergoing the ion implant step to form N⁻source/drain (S/D) lightly doped regions in accordance with the present invention;

[0041] FIG. 16 is a schematic cross-sectional illustration of the structure of FIG. 15 after the formation of spacers in accordance with the present invention;

[0042] FIG. 17 is a schematic cross-sectional illustration of the structure of FIG. 16 undergoing an ion implant step to form source and drain regions in accordance with the present invention;

[0043] FIG. 18 is a schematic cross-sectional illustration of the structure of FIG. 17 after undergoing the ion implant step to form source and drain regions in accordance with the present invention;

[0044] FIG. 19 is a schematic cross-sectional illustration of the structure of FIG. 18 after formation of an oxide layer over the structure in accordance with the present invention; and

[0045] FIG. 20 is a schematic cross-sectional illustration of the structure of FIG. 19 after the oxide layer has been polished down to the surface level of the gate in accordance with the present invention.

[0046] The present invention relates to a MOSFET device structure which facilitates mitigation of junction capacitance and/or floating body effects, and a method for making the same. The MOSFET device of the present invention exhibits faster performance, lower power consumption and less device hysteresis than many conventional MOSFET devices. The present invention accomplishes this by providing a capacitive coupling of a gate region and a body region of the MOSFET device to configure the device as a dynamic threshold metal oxide field effect transistor (DTMOS). The capacitive coupling forms a capacitive voltage divider with the junction capacitance of the device to allow the device to operate above 0.6-0.8 volts. The present invention will now be described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. Although the present invention is described primarily in connection with an SOI MOSFET device structure, the present invention may be employed in connection with bulk MOSFET device structures as well. It should be understood that the herein description of the present invention is merely illustrative and that it should not be taken in a limiting sense.

[0047] FIG. 1ais a schematic cross-sectional side view illustration of an SOI MOSFET device structure 50 across a central region in accordance with the present invention. The device structure 50 includes a base 60 comprising silicon, for example. The base 60 provides mechanical support for the device structure 50, and is of a thickness suitable for providing such support. A dielectric layer 64 (e.g., SiO₂, Si₃N₄) is formed over the base 60. The thickness of the dielectric layer 64 is preferably within the range of 1000 Å to 5000 Å. A top silicon layer 70 is shown formed over the dielectric layer 64, and the top silicon layer preferably has a thickness within the range of 500 Å to 2000 Å. The top silicon layer 70 becomes the active region for device fabrication. The device structure 50 includes a gate 90 and n-type channel 94 (FIG. 1b), and a gate oxide layer 100 formed between the gate 90 and the channel 94. An oxide layer 230 serves to protect the device 50 from contaminants, etc.

[0048] FIG. 1bis a schematic cross-sectional illustration of the SOI MOSFET device structure 50 along the lines A—A of FIG. 1ain accordance with the present invention. The device structure 50 is an NMOS type device and further includes a N⁺drain region 80, an N⁺source region 82, a N⁻lightly doped drain extension region 84, and an N⁻lightly doped source extension region 86. Alongside the N⁺drain region 80 and N⁻lightly doped drain extension region 84, and the N⁺source region 82, and the N⁻lightly doped source extension region 86 is a heavily doped P⁺⁺region 110 (see FIGS. 1aand 1c). FIG. 1cillustrates a cross-sectional view of the device structure 50 along the lines B—B of FIG. 1a. The heavily doped region 110 forms a capacitance between the gate 90 and the body 120 which couples the gate 90 to the body 120 and forms a capacitive voltage divider with the junction capacitance of the device. The heavily doped region 110 also facilitates voltage control of floating body effects (e.g., kink effect and hysteresis effects) of the device 50. As can be seen from FIG. 1a, the heavily doped P⁺⁺region 110 runs under the gate 90 alongside the transistor N⁺drain region 80 and N⁻lightly doped drain extension region 84. The heavily doped P⁺⁺region 110 also runs under the gate 90 alongside the transistor N⁺source region 82 and N⁻lightly doped source extension region 86.

[0049] Lowering the doping concentration of the source/drain regions reduces the junction capacitances between the drain/body and source/body interfaces. The junction capacitance is related to the doping concentrations of the regions forming the junction, as can be seen from the following formula:

[0050] C_j=εA[(q/2ε(V_o−V))(N_aN_d/(N_a⁺N_d)]^½

[0051] wherein A represents the cross sectional area of the source/body and drain/body interfaces, N_drepresents the number of donors in the source and drain regions, respectively, and N_arepresents the number of acceptors in the body.

[0052] The heavily doped region is doped with a P⁺type dopant (e.g., Boron) and at a dopant concentration higher than the N⁺dopant concentration of the source/drain regions 80, 82 and the P⁺body region 120. As a result, the heavily doped region 110 forms a capacitance between the gate 90 and the body 120 which couples the gate 90 to the body 120 and serves to act as a voltage divider. This allows use of the NMOS device as a DTMOS device at voltages higher than a diode drop.

[0053] In one specific embodiment of the present invention, heavily doped region 110 preferably include a boron implant having a dose within the range of 1−10¹⁸to 1×10²⁰atoms/cm²at an energy range of about 1 KeV to about 100¹⁸KeV. The lightly doped source/drain extension regions include an arsenic implant having a dose concentration in the range of 1×10¹⁴to 1×10¹⁶atoms/cm²and implanted at an energy range of about 50 KeV to about 200 KeV. The source and drain regions 80, 82 include an arsenic or phosphorus implant having a dose within the range of 1×10¹⁷to 1×10¹⁷atoms/cm²at an energy range of about 50 KeV to about 200 KeV. It will be appreciated that any suitable dose and energy range and implant may be employed to carry out the present invention. The p-type body 120 includes a P⁺implant (e.g., Boron) having a dose within the range of 1×10¹⁰to 1×10¹⁴atoms/cm².

[0054] FIG. 1dillustrates a schematic diagram of an equivalent circuit of the SOI MOSFET structure 50. The equivalent circuit includes a MOSFET 140 having a gate region 156, a body region 158, a source 144 and a drain 152. A capacitor 146 labeled as C_DTis connected from the body region 158 to the gate region 156 via a contact point 150. The capacitor 146 combines with the junction capacitance Cj of the MOSFET 140 to form a voltage divider between voltage applied to the gate region of the MOSFET 140 and the voltage level at the body region 158 of the MOSFET structure 50. The voltage level at the body region 158 is proportional to the voltage applied at the gate region 156 and is governed by the following equation:

[0055] ΔV_B=(C_DT/C_DT⁺Cj)*ΔV_G

[0056] where Cj is the junction capacitance of the MOSFET 140. FIGS. 1e-1fillustrate the relationship of V_Gand V_Bover time where V_B(max) is proportional to V_Gbut less than V_DDas determined by the capacitive ratio illustrated in the above equation.

[0057] Turning now to FIGS. 2-20, fabrication steps in connection with forming the structure 50 of FIG. 1aare discussed. FIGS. 2-11 illustrates the rear view cross-section as shown in FIG. 1band FIGS. 12-20 illustrate the front view cross-section as shown in FIG. 1a. FIG. 2 illustrates a basic SOI structure in its early stages of fabrication. The structure includes the silicon base 60, the silicon oxide layer 64 and the top silicon layer 70. This basic structure is formed preferably via a SIMOX (Separation by Implantation of Oxygen) process. The basic steps of the SIMOX process involve implanting oxygen beneath the surface of a silicon wafer. An annealing step is next performed to coalesce the implanted oxygen atoms into a uniform layer of Si0₂. Sometimes, epitaxial silicon may be grown atop the silicon to satisfy specific device requirements, but with or without an epitaxial layer, the top surface film 70 becomes the active region for device fabrication. The buried oxide layer 64 is typically 0.1 to 0.5 μm thick and exhibit almost complete incorporation of the implanted oxygen. Typical implant energies range from 150 to 200 keV, while the oxygen dose may vary from 1 to 2E18 cm⁻². The top silicon film 70 thickness as well as the variation thereof with respect to the oxide layer 64 thickness is a function of the implant energy as well as the rate of surface silicon sputtering during the implant process.

[0058] A second significant step in the SIMOX process is high temperature annealing. Such annealing is typically performed at temperatures greater than 1250° C. for several hours to coalesce the implanted oxygen and achieve solid state recrystallization of the top (superficial) silicon layer 70 from the surface downward.

[0059] FIGS. 3-8 illustrate process steps associated with fabrication of isolation regions of the MOSFET device 50 in accordance with the present invention. The present process relates to shallow trench isolation (STI), which involves etching a trench into the top silicon layer 70 and filling the trench with an isolation material. Local oxidation of silicon (LOCOS) isolation processes generally occupy a great deal of wafer surface area, and thus STI provides for an alternative isolation technique.

[0060] FIG. 3 illustrates a pad oxide layer 160 and a layer of nitride 162 formed on the surface of the top silicon layer 70. The pad oxide layer 160 has a thickness of approximately 200 Å and may be thermally grown at a temperature of approximately 900° C. for a period of 40 minutes. The nitride layer 162 is deposited on the surface of the pad oxide layer 160 via a chemical vapor deposition (CVD) process to a thickness of approximately 2000 Å. A conventional photoresist process is then utilized to pattern and etch the nitride layer 162 and the pad oxide layer 160 in order to result in the structure shown in FIG. 4. Such photoresist process includes use of an STI mask which defines isolation regions 168. The isolation regions 168 are located on the substrate 60 at positions which will interpose active regions later formed on the top silicon layer 70.

[0061] Next, as represented in FIG. 5, a silicon etch is performed so as to form shallow trenches 170 within the top silicon layer 70 at the isolation regions 168. In particular, a trench resist material (not shown) is applied to cover the structure and is then patterned to expose the isolation regions 168. The shallow trenches 170 are then etched into the top silicon layer 70 using suitable techniques. The trench resist material is thereafter stripped so as to result in the structure shown in FIG. 5.

[0062] Following the formation of the trenches 170 via the silicon etch, a layer of oxide material 174 is formed on the structure using high density plasma chemical vapor deposition (HDPCVD) in order to fill fully the isolation regions 170 with the oxide material 174 as shown in FIG. 6. As is known, HDPCVD is a self-planarizing process which facilitates reduction of the chemical mechanical polishing (CMP) times required in subsequent steps. (See, e.g., Pye, J. T.et al., High-density plasma CVD and CMP for0.25-μm intermetal dielectric processing, Solid State Technology, December 1995, pgs. 65-71). Following deposition of the oxide material 174, the oxide material 174 is polished via CMP down to the surface level of the nitride layer 162 as shown in FIG. 7. Consequently, the insulative oxide material 174 in the trenches 170 remains. The upper surface of the oxide material 174 is substantially even with the upper surface of the nitride layer 162.

[0063] As is represented in FIG. 8, the nitride layer 162 and pad oxide layer 160 are stripped away using a suitable stripping process. The strip process also results in the top surface of the oxide material 174 being etched to a level substantially even with the surface of the top silicon layer 70. Thus, the formation of the shallow isolation trenches 170 is substantially complete in relevant part.

[0064] Turning now to FIGS. 9-20, process steps in connection with completion of the MOSFET device 50 in accordance with the present invention are described. Although the present invention is described in the context of fabricating an NMOS type device it is to be appreciated that the present invention as applicability to a variety of transistor devices including PMOS type devices. The present description will enable those skilled in the art to practice the invention with respect to a vast number of different types of transistor devices which are intended to fall within the scope of the invention as defined by the hereto appended claims.

[0065] The top silicon layer 70 is of a p-type and the trenches 170 serve as isolation barriers to define active regions. FIG. 9 shows the formation of the p-type body 120 by masking a portion of the top silicon layer 70 with a photoresist layer (not shown) and implanting p-well dopants 180 to provide the p-type body 120. Turning now to FIG. 10, a second implant step 190 is performed to implant P⁺⁺implants at a higher dose than the p-type body implant of step 190 to achieve the highly doped region 110. A special mask is employed to ensure that the p-type implant only implants the structure in a specific region of the device 50. The P⁺⁺implant of step 190 is preferably Boron at a dopant concentration within the range of about 1×10¹⁸-1×10¹⁹atoms/cm³. The P⁺⁺implant provides a capacitive coupling of a gate region and a body region of the MOSFET device to configure the device as a dynamic threshold metal oxide field effect transistor (DTMOS). FIG. 11 illustrates the capacitance portion of the structure 50 completed in relevant part.

[0066] FIGS. 12-20 illustrate a front view cross-section of the formation of the transistor portion of the device structure 50 as shown in FIG. 1b. FIG. 12 illustrates the thin gate oxide material 100 being laid down on the top silicon layer 70 between the shallow trenches 170. The thin gate oxide material 100 is formed to have a thickness within the range of about <40 Å. Preferably, the thin gate oxide material 100 includes SiO₂which has a substantially low dielectric constant. However, it is to be appreciated that any suitable material (e.g., Si₃N₄) for carrying out the present invention may be employed and is intended to fall within the scope of the present invention. Alternatively, the thin gate oxide material 100 can be formed prior to the formation of the heavily doped region 110.

[0067] Thereafter as shown in FIG. 13 the gate 90 is formed between the shallow trenches 170 over the thin gate oxide material 100. The gate 90 is made of polysilicon. The gate 90 has a thickness within the range of about 1000 to 2000 Å, and the thickness of the gate 90 is chosen so as to account for any subsequent polishing that might be performed. It will be appreciated of course that the thickness of the thin gate oxide material 100 and the gate 90 may be tailored as desired and the present invention intends to include any suitable range of thicknesses thereof for carrying out the present invention. Excess gate oxide material 100 is removed as is conventional.

[0068] FIG. 14 illustrates an n-region first ion implant step 200. A capacitor mask is employed to protect the highly doped P⁺⁺regions during the n-type implants. A N⁻implant 200 is used to form n-channel transistor lightly doped regions 84 and 86 (FIG. 15) which are self-aligned with the gate 90. In the preferred embodiment, this implant step may be an arsenic implant for example having a dose in the range of 1×10¹⁴to 1×10¹⁶atoms/cm²and an energy range of about 50KeV to about 200 KeV. It will be appreciated that any suitable dose and energy range and implant may be employed to carry out the present invention.

[0069] Next, after the step of implanting arsenic, an optional nitrogen implant step may be performed as part of implant step 200. Nitrogen may be added via implantation in the lightly doped regions 84 and 86. The nitrogen implant may be applied at a dose of 1×10¹⁴to 5×10¹⁵atoms/cm²with an energy range of about 50 KeV to 200 KeV. It will be appreciated that although in the preferred embodiment the nitrogen implant step is performed after the boron implantation, the nitrogen implantation may be performed prior to the boron implantation.

[0070] The implantation of the nitrogen as represented results in reduced series resistance and hot carrier effects without significantly increasing S/D extension overlap. Contrary to conventional MOS fabrication techniques where increasing dopant concentration results in lower sheet resistance, the nitrogen implantation does not result in a deeper junction as a result of the increase in dopant. On the other hand, if the arsenic dopant dose is increased in order to lower sheet resistance, a deeper junction would result. The deeper junction may result in bad roll-off, make the MOS device more difficult to control, and possibly result in punch through effects. However, unlike conventional techniques, the nitrogen implantation results in reduced series resistance. Thus, this step provides for reducing series resistance without the negative consequences (e.g., hot electron carrier and punch through effects) associated with conventional techniques.

[0071] Furthermore, the nitrogen implantation does not result in any significant increase in the amount of diffusion of the S/D extension into the gate. When an implant is provided in the S/D extension region the implant not only spreads vertically there is also horizontal spreading of the implant which is known as S/D extension overlap into the gate. The use of nitrogen implant does not result in any significant increase in S/D extension overlap as compared to conventional dopants.

[0072] After the implant step 200, the spacers 92 are formed along sidewalls of the gate 90. To accomplish this step, a spacer material layer (not shown) may be formed over the top silicon layer 70. The spacer material layer may be formed by depositing tetraethoxysilane (TEOS) oxide, silicon dioxide or the like over the surface of the top silicon layer 70. The spacer material is then anisotropically etched to form the spacers 92 on the sidewalls of the gate 90, for example. An etchant which selectively etches the spacer material layer (e.g., etches the spacer material layer at a faster rate than the top silicon layer 70), may be used to etch the spacer material layer until only the spacers 92 remain at the sidewalls of the gate 90 as shown in FIG. 16.

[0073] After the formation of the spacers 92 another ion implant step 210 is performed as shown in FIG. 17. A capacitor mask is employed to protect the highly doped P⁺⁺regions during the n-type implants. An N⁺implant 210 is performed to form N⁺source region 80 and N⁺drain region 82 (FIG. 18) in portions of the lightly doped regions, respectively. The spacers 92 serve as masks to prevent ion implantation in the portions of lightly doped regions underlying the spacers 92. These protected portions of the lightly doped regions are the respective lightly doped drain (LDD) region 84 and lightly doped source (LDS) region 86 of the device 50.

[0074] Turning now to FIG. 19, an oxide layer 230 is deposited over the MOSFET device 50. The oxide layer 230 is then polished via a chemical mechanical polish (CMP) down to the surface level of the gate 90 as shown in FIG. 20. The upper surface of the oxide layer 230 is thus substantially even with the upper surface of the gate 50. Consequently, the oxide layer 230 serves to mask the entire MOSFET device 50 except for the exposed gate 90. The spacers 92 are no longer depicted because they are of the same material as the oxide layer 230. Thus, the MOSFET device 50 is complete in relevant part.

[0075] Substantially the same fabrication methodology may be employed in the fabrication of such an n-channel device as a bulk device as compared to the discussed SOI type device. One skilled in the art could readily tailor the above steps to form such n-channel devices based on the discussion herein, and therefore further discussion related thereto is omitted for sake of brevity.

[0076] What has been described above are preferred embodiments of the present invention. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the present invention, but one of ordinary skill in the art will recognize that many further combinations and permutations of the present invention are possible. Accordingly, the present invention is intended to embrace all such alterations, modifications and variations that fall within the spirit and scope of the appended claims.