DUAL POLY DEPOSITION AND THROUGH GATE OXIDE IMPLANTS
Dopants are implanted at relatively high energies into an unmasked first region of a semiconductor substrate through a thin layer of gate electrode material and a gate dielectric layer. Lower energy dopants are then implanted into the thin layer of gate electrode material. The first region is then masked off, and the process is repeated in a previously masked, but now unmasked, second region of the semiconductor substrate. A second (and usually thicker) layer of gate electrode material is then formed over the thin layer of gate electrode material. The layer of thick gate electrode material, the layer of thin gate electrode material and the layer of gate dielectric material are patterned to form one or more gate structures over the doped regions of the substrate. Source and drain regions are formed in the substrate regions adjacent to the gate structures to establish one or more MOS transistors.
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The present invention relates generally to semiconductor processing, and more particularly to performing a dual poly deposition, implanting dopants through a gate oxide, and dual pre-pattern poly doping.
BACKGROUND OF THE INVENTIONIt can be appreciated that semiconductor processing can comprise hundreds of steps, during which many copies of an integrated circuit can be formed on a single semiconductor substrate or workpiece, generally known as a wafer. Generally, such processes involve creating several layers on and in the substrate that ultimately form a complete integrated circuit. This layering process can create electrically active regions in and on the semiconductor wafer surface. In a metal-oxide-semiconductor (MOS) transistor, for example, a gate structure is created, which can be energized to establish an electric field within a semiconductor channel, by which current is enabled to flow between a source region and a drain region within the transistor. The source and drain regions facilitate this conductance by virtue of containing a majority of hole (p type) or electron (n type) carriers.
It can also be appreciated that there is an ongoing desire to streamline fabrication processes to reduce fabrication times and cut costs. Likewise, it is desirable to enhance transistor operation. Accordingly, techniques that allow MOS transistors to be fabricated in a cost effective manner, while also improving the functionality of resulting transistors are desirable.
SUMMARY OF THE INVENTIONThe following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended neither to identify key or critical elements of the invention nor to delineate the scope of the invention. Rather, its primary purpose is merely to present one or more concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
Implantations of one or more types of dopants to form one or more MOS transistors are performed through a thin layer of gate electrode material and a layer of gate dielectric material. Additionally, dopants are implanted into the thin layer of gate electrode material, where at least some of this layer serves as a lower portion of a gate electrode of a transistor. Locating the dopants in this part of the gate electrode enhances the operation of the transistor by mitigating the likelihood of poly depletion, for example. These two implantations are performed using the same mask which reduces the number of masking steps needed and thereby streamlines the fabrication process.
To the accomplishment of the foregoing and related ends, the following description and annexed drawings set forth in detail certain illustrative aspects and implementations of the invention. These are indicative of but a few of the various ways in which one or more aspects of the present invention may be employed. Other aspects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the annexed drawings.
An exemplary methodology 100 for forming one or more MOS transistors is illustrated in
The methodology 100 begins at 102 wherein a layer of gate dielectric material 202 is formed over a semiconductor substrate 200 (
A first layer of gate electrode material 204 is then formed over the layer of gate dielectric material 202 at 104 (
At 106 part of the substrate is masked off so that one or more dopants can be selectively implanted into a first unmasked area of the substrate. For example, a first resist 206 can be formed over the entire substrate 200 and patterned in accordance with lithographic techniques so that an exposed portion 210 of the substrate 200, wherein a first type of transistor is to be formed, can be doped to form a first well therein (
Accordingly, at 108 a first implantation 216 is performed to implant dopants into the first unmasked region 210 of the substrate 200 (
At 110, a second implantation 220 is performed to implant dopants having the first electrical conductivity type into a first region 222 of the first layer of gate electrode material 204 overlying the first region 210 of the substrate 200, where these dopants generally do not penetrate into and/or are stopped by the layer of gate dielectric material 202 (
A third implantation 236 is thus performed at 114 to establish the second well 232 in the second region 230 of the substrate 200 (
At 116, a fourth implantation 240 is performed to implant dopants having the second electrical conductivity type into a second region 242 of the first layer of gate electrode material 204 overlying the second region 230 of the substrate 200, where these dopants generally do not penetrate into and/or are stopped by the layer of gate dielectric material 202 (
The second patterned resist 226 is then stripped and a second layer of gate electrode material 250 is formed over the first layer of gate electrode material 204 at 118 (
The second layer of gate electrode material 250, the first layer of gate electrode material 204 and the layer of gate dielectric material 202 are then patterned at 120 to form one or more gate structures 252, 254 over the first 210 and second 230 regions, respectively, of the substrate 200 (
Implants to form respective source 262, 264 and drain 266, 268 regions are then performed, wherein any suitable masks and implantation processes may be used in forming the source and drain regions to achieve desired transistor types (
The transistors 282, 284, and more particularly the first 210 and second 230 regions of the substrate 200 and the first 212 and second 232 wells contained therein, are electrically separated from one another by one or more (previously formed) isolation (e.g., dielectric) regions 290 formed in the substrate 200. In the illustrated example, a single transistor 282 is formed over the first region 210 and a single transistor 284 is formed over the second region 230. It will be appreciated, however, that any number of transistors (including none/zero transistors) can be formed in either of the different regions 210, 230. Once the source/drain regions are formed, the methodology 100 advances to 122, and ends thereafter, wherein further back end processing can be performed at 122, such as the formation and/or patterning of one or more additional conductive and/or non-conductive layers.
It can be appreciated that the gate structures 252, 254 thus comprise respective gate electrodes 222, 250 and 242, 250 and gate dielectrics 202. It will be appreciated that the gate electrodes yield a contact area or surface that provides a means for applying a voltage to the transistors 282, 284 or otherwise biasing the transistors 282, 284. The gate dielectrics generally serve to prevent large ‘leakage’ currents from flowing from the conductive gate electrodes into the conductive channel regions 270, 272 when voltages are applied to electrodes, while also allowing applied gate voltages to set up electric fields within the channel regions 270, 272 in a controllable manner.
It will be appreciated that the operation of the transistors 282, 284, and in particular current flowing therein, is also a function of the doping of the gate electrodes. In some instances the gate electrodes run out of or become depleted of dopants or electrical carriers. If this happens, the transistors do not function as desired. This depletion is often a function of the location of the dopants within the electrodes, and more particularly because the dopants are located too far away from the channel regions. Accordingly, because dopants are implanted (e.g., at 110/220 and 116/240) in the thin layer of gate electrode material 204, and thus closer to the channel regions 270, 272, transistors fashioned as described herein function in a more desirable manner than transistors that do not have a high concentration of dopants in a bottom portion (e.g., 222, 242) of their gate electrodes (at least relative to the upper portions of their gate electrodes). In particular, transistors fashioned as described herein are less likely to experience gate electrode (e.g., polysilicon) depletion.
Additionally, forming transistors as described herein streamlines the fabrication process, at least, by reducing the number of masks that are needed. In particular, since dopants are implanted through the gate dielectric 202 and first gate electrode 204 at 108/216 and 114/236 to form the first 212 and second 232 wells, merely two masks 206, 226 are needed. However, more masks (e.g., four) are needed if the wells are not formed by implanting through the gate dielectric 202 and the first gate electrode 204 layer. For example, first and second masks would be needed to establish the first 212 and second 232 wells. Additionally, third and fourth masks would also be needed to then dope the single layer of gate electrode material (e.g., where one thick layer of gate electrode material is utilized as opposed to a thin layer 204 and a thicker layer 250 as described herein). Further, since the dopants are applied directly to the bottom portions 222, 242 of the transistors 282, 284, an annealing or other heat treatment process is not needed to force the dopants down toward the bottom of the single thick gate electrode layer, thus substantially lessening a required thermal budget. Also, implanting directly into the lower portion of the gate electrode provides a much greater degree of control over where the dopants ultimately end up in the gate electrode, particularly as compared to heating the dopants to move them around.
Implanting dopants through the (thin) layer of gate electrode material 204 to form the first 212 and second 232 wells also allows a much lower energy to be used to implant the dopants, as compared to having to implant the dopants thorough a single thicker layer of gate electrode material. Using lower energy implants is more desirable as it is less destructive (e.g., to the gate dielectric layer). Implanting through the thinner layer of gate electrode material 204 also allows more control over the implantation process, such as to achieve a retrograde profile, for example, where a retrograde profile refers to a concentration of dopants that increases in relation to implantation depth (e.g., dopant concentration increases as depth increases). It will be appreciated that achieving a retrograde profile is desirable since such a dopant distribution allows for desired device performance.
In one example, a retrograde profile in a transistor fashioned as described herein comprises a dopant of Arsenic at a concentration of between about 1e18/cm3 and about 2e18/cm3 at a depth of between about 200 Angstroms and about 400 Angstroms and having a concentration that is reduced to between about 1e18/cm3 and about 3e17/cm3 as the implantation depth is reduced to between about 0 Angstroms and about 100 Angstroms. Stated another way, a higher dopant concentration is produced deeper in the substrate, while a lighter dopant concentration is produced at shallower locations in the substrate. The deeper dopant concentration is thus between about 1e18/cm3 and about 2e18/cm3 at a depth of between about 200 Angstroms and about 400 Angstroms, whereas the dopant concentration is merely between about 1e18/cm3 and about 3e17/cm3 at a depth of between about 0 Angstroms and about 100 Angstroms. It can be appreciated that a retrograde profile is difficult to achieve when implanting through a thick layer of gate electrode material since the dopants are slowed down by the thickness of the layer.
Implanting through the (thin) layer of gate electrode material 204 also allows more uniform implantations to be achieved (e.g., a consistent retrograde profile across the substrate). Implanting through a thick layer of gate electrode material can result in non-uniform implantations as the trajectory of the dopants can be interfered with by the by the thick layer. Additionally, a thicker layer of gate electrode material is more likely to embody non-uniformities and/or undulations than the thin layer of gate electrode material 204 which can be formed more precisely. And, such imperfections may be reflected in or transferred to the implanted dopants. For example, dips in a thick layer of gate electrode material may allow dopants to be implanted more deeply, while elevated portions of a thick layer of gate electrode material may cause dopants to be implanted less deeply.
It will be appreciated that while reference is made throughout this document to exemplary structures in discussing aspects of methodologies described herein (e.g., those structures presented in
Although the invention has been shown and described with respect to one or more implementations, equivalent alterations and modifications will occur to others skilled in the art based upon a reading and understanding of this specification and the annexed drawings. The invention includes all such modifications and alterations and is limited only by the scope of the following claims. In addition, while a particular feature or aspect of the invention may have been disclosed with respect to only one of several implementations, such feature or aspect may be combined with one or more other features or aspects of the other implementations as may be desired and advantageous for any given or particular application. Furthermore, to the extent that the terms “includes”, “having”, “has”, “with”, or variants thereof are used in either the detailed description or the claims, such terms are intended to be inclusive in a manner similar to the term “comprising.” Also, the term “exemplary” is merely meant to mean an example, rather than the best. It is also to be appreciated that features and/or elements depicted herein are illustrated with particular dimensions relative to one another (e.g., layer to layer dimensions, orientations and/or scaling) for purposes of simplicity and ease of understanding, and that actual dimensions of the elements may differ substantially from that illustrated herein.
Claims
1. A MOS transistor comprising:
- a gate structure, comprising: a gate dielectric formed over a semiconductor substrate; a first gate electrode formed over the gate dielectric; and a second gate electrode formed over the first gate electrode;
- a source region formed in the semiconductor substrate adjacent to one side of the gate structure; and
- a drain region formed in the semiconductor substrate adjacent to the other side of the gate structure.
2. The transistor of claim 1, where the first gate electrode comprises a dopant of Arsenic at a concentration of between about 1e15/cm2 and about 4e15/cm2.
3. The transistor of claim 2, where the first gate electrode is formed to a thickness of between about 80 Angstroms and about 300 Angstroms and the second gate electrode is formed to a thickness of between about 800 Angstroms and about 1000 Angstroms.
4. The transistor of claim 3, where the source and drain regions are formed in a well region in the semiconductor substrate, where the well region has a retrograde profile of a dopant of Boron at a concentration of between about 1e18/cm3 and about 2e18/cm3 at a depth of between about 200 Angstroms and about 400 Angstroms and a concentration of between about 3e17/cm3 and about 1e18/cm3 at a depth of between about 0 Angstroms and about 100 Angstroms.
Type: Application
Filed: Sep 4, 2008
Publication Date: Dec 25, 2008
Applicant: TEXAS INSTRUMENTS INCORPORATED (Dallas, TX)
Inventors: Shaofeng Yu (Plano, TX), Shyh-Horng Yang (Plano, TX)
Application Number: 12/204,072
International Classification: H01L 29/78 (20060101);