# Patent

## Title
Embedded transistor

## Abstract

An embedded transistor for an electrical device, such as a DRAM memory cell, and a method of manufacture thereof is provided. A trench is formed in a substrate and a gate dielectric and a gate electrode formed in the trench of the substrate. Source/drain regions are formed in the substrate on opposing sides of the trench. In an embodiment, one of the source/drain regions is coupled to a storage node and the other source/drain region is coupled to a bit line. In this embodiment, the gate electrode may be coupled to a word line to form a DRAM memory cell.

## Background
This application is a divisional of co-pending U.S. patent application Ser. No. 13/273,012, filed on Oct. 13, 2011, entitled “Embedded Transistor,” now U.S. Pat. No. 8,853,021, which application is hereby incorporated herein by reference.


BACKGROUND
Generally, complementary metal oxide-semiconductor (CMOS) transistors include a gate electrode and a gate dielectric, which are formed on a substrate (usually a silicon semiconductor substrate). Lightly doped drains are formed on opposing sides of the gate electrode by implanting N-type or P-type impurities into the substrate. An oxide liner and one or more implant masks (commonly referred to as spacers) are formed adjacent the gate electrode, and additional implants are performed to complete the source/drain regions. Current flowing through the source/drain regions may then be controlled by controlling the voltage levels applied to the gate electrode.
Reduction in the size of CMOS transistors has provided continued improvement in speed, performance, circuit density, and cost per unit function over the past few decades. As the gate length of the conventional bulk MOSFET is reduced, the source and drain increasingly interact with the channel and gain influence on the channel potential. Consequently, a transistor with a short gate length suffers from problems related to the inability of the gate to substantially control the on and off states of the channel.



BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete understanding of the present disclosure, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
FIGS. 1-8 illustrate various intermediate stages in the manufacturing of an embedded transistor in accordance with an embodiment;
FIG. 9 illustrates a plan view of a memory cell utilizing an embedded transistor in accordance with an embodiment; and
FIGS. 10A and 10B are cross-sectional views of the memory cell illustrated in FIG. 9.



DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
The making and using of the embodiments are discussed in detail below. It should be appreciated, however, that the present disclosure provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the disclosure, and do not limit the scope of the disclosure. Throughout the various views and illustrative embodiments of the present disclosure, like reference numbers are used to designate like elements.
Referring first to FIG. 1, a substrate 110 is provided with a first patterned mask 112 formed thereon. The substrate 110 may comprise any semiconductor material and may comprise known structures including a graded layer or a buried oxide, for example. In an embodiment, the substrate 110 comprises bulk silicon that may be undoped or doped (e.g., p-type, n-type, or a combination thereof). Other materials that are suitable for semiconductor device formation may be used. In an embodiment, however, the substrate 110 is bulk silicon.
The first patterned mask 112 is formed to pattern the underlying materials, such as the underlying substrate 110. In an embodiment, the first patterned mask 112 comprises a photoresist material that has been masked, exposed, and developed. Generally, the photoresist material is deposited, irradiated (exposed) and developed to remove a portion of the photoresist material, thereby defining a pattern as illustrated in FIG. 1. The remaining photoresist material protects the underlying material from subsequent processing steps, such as etching.
Also shown in FIG. 1 is an optional hard mask 114. The hard mask 114 is a protective layer to prevent the underlying structures, such as the substrate 110, from being removed during an etching process. In some situations, a mask in addition to the first patterned mask 112 is desirable due to the materials to be patterned, the duration of the etch process, the types of etchants used, and the like. In an embodiment in which the substrate 110 is a silicon substrate, one such suitable hard mask 114 comprises an oxide layer, such as a silicon oxide layer, and an overlying nitride layer, such as a silicon nitride (Si3N4) layer. The oxide layer may be formed by any oxidation process, such as wet or dry thermal oxidation in an ambient comprising an oxide, H2O, NO, or a combination thereof, or by chemical vapor deposition (CVD) techniques using tetra-ethyl-ortho-silicate (TEOS) and oxygen as a precursor. The oxide layer may also be formed, for example, by an in-situ steam generation (ISSG) process in an ambient environment of O2, H2O, NO, a combination thereof, or the like. In an embodiment, the oxide layer is about 50 Å to about 100 Å in thickness. The nitride layer may be formed using CVD techniques using silane and ammonia as precursor gases. The nitride layer may be patterned using CHF3 plasma, and the oxide layer may be patterned using CF4 plasma.
One of ordinary skill in the art will appreciate that other mask materials and/or structures may be used to form either or both of the first patterned mask 112 and the hard mask 114. For example, other materials, a single layer, three or more layers, or the like may be used. In an alternative embodiment, the hard mask may comprise a single silicon nitride layer without an underlying oxide layer.
FIG. 2 illustrates the substrate 110 after the substrate has been patterned in accordance with an embodiment. The substrate 110 may be patterned by performing one or more etching steps, thereby forming trenches 2161-2165 (collectively referred to as trenches 216) having fins 218 interposed between adjacent ones of the trenches 216. The substrate 110 may be etched by, for example, HBr/O2, HBr/Cl2/O2, or SF6/Cl2 plasma. As will be discussed in greater detail below, the fins 218 will form source/drain regions of a transistor, while alternating ones of the trenches will form the gate electrodes of the transistor. Other ones of the trenches will form an isolation structure, e.g., shallow trench isolations (STIs).
In the embodiment illustrated in FIG. 2, the trenches 216 may have a depth D1 (and hence the height of the fins 218) of about 1,000 Å to about 4,000 Å, and the fins 218 may have a width W1 of about 100 Å to about 800 Å. While the width W1 of the fins 218 are illustrated in this embodiment are the same, other embodiments may utilize fins 218 of varying widths. As noted above, subsequent processing forms source/drain regions in the upper portions of the fins 218. Thus, the size (e.g., the width and height of the fins 218) may be adjusted to achieve the desired electrical characteristics of the transistor. Moreover, it should be noted that the fins on the same wafer may have different widths and depths.
Additionally, a width W2 of the trenches may also vary. As noted above, the trenches will become the gate electrodes and isolation trenches. As such, the width of the trenches may be adjusted to vary the gate length and the isolation characteristics. For example, it may be desirable in some embodiments to provide wider isolation trenches as compared to the trenches for the gate electrode to provide greater isolation characteristics between adjacent devices. In other embodiments, a wider trench for the gate electrode may be desirable.
Also illustrated in FIG. 2 is the removal of the first patterned mask 112 (see FIG. 1). The first patterned mask 112 may be removed, for example, by an O2 plasma dry strip and a mixture of concentrated sulphuric acid and hydrogen peroxide.
Referring now to FIG. 3, a first dielectric material 320 is formed over the substrate 110, substantially filling the trenches 216. In an embodiment, the first dielectric material 320 comprises a silicon oxide layer that may be formed by a high-density plasma CVD deposition process using SiH4 and O2 mixture.
As illustrated in FIG. 3, the first dielectric material 320 is planarized to a top surface of the substrate 110 in accordance with an embodiment. The first dielectric material 320 may be planarized, for example, by using a chemical-mechanical polishing (CMP) process using an oxide slurry wherein the substrate 110 acts as a stop layer.
FIG. 4 illustrates removal of the first dielectric material 320 from select ones of the trenches 216, such as trenches 2162 and 2164. In an embodiment, the first dielectric material 320 may be selectively removed from trenches 2162 and 2164 by forming and patterning a mask layer (not shown) to protect the first dielectric material 320 in trenches 2161, 2163, and 2165 while leaving the dielectric material in trenches 2162 and 2164 exposed. The mask may be patterned using photolithography techniques similar to those discussed above with reference to etching the substrate 110 as illustrated in FIGS. 1 and 2. For example, a photoresist material may be formed, exposed according to a desired pattern (e.g., exposing trenches 2162 and 2164), and developed. Additionally, a hard mask such as that discussed above may also be used.
In an embodiment in which the first dielectric material 320 is a silicon oxide and the substrate 110 is silicon, the first dielectric material 320 may be removed using an anisotropic dry etch process using an etchant having a high etch selectivity between the substrate 110 and the first dielectric material 320, such as CF4 or C2F6. In this manner, substrate 110 is relatively unaffected while etching or removing the first dielectric material 320.
FIG. 5 illustrates the substrate 110 after a gate insulator layer 526 is formed along the surfaces of the trenches 2162 and 2164 and a gate electrode material 528 is formed within the trenches 2162 and 2164, in accordance with an embodiment. Generally, the gate insulator layer 526 prevents electron depletion between the source/drain regions and the gate electrode. In an embodiment, the gate insulator layer 526 comprises an oxide layer formed by an oxidation process, such as wet or dry thermal oxidation in an ambient comprising an oxide, H2O, NO, or a combination thereof, an in-situ steam generation (ISSG) process in an ambient environment of O2, H2O, NO, a combination thereof, or the like, or by chemical vapor deposition (CVD) techniques using tetra-ethyl-ortho-silicate (TEOS) and oxygen as a precursor. Other materials including, high k dielectric materials, such as HfO2, HfSiO2, ZnO, ZrO2, Ta2O5, Al2O3 and the like, and other processes, such as Atomic Layer Deposition (ALD), Atomic Vapor Deposition (AVD), and the like, may also be used. In an embodiment, the gate insulator layer 526 has a thickness between about 20 Å and about 50 Å. It should be noted that FIG. 5 illustrates that the gate insulator layer 526 does not extend over the first dielectric material 320 for illustrative purposes only. Whether the gate insulator layer 526 extends over the first dielectric material 320 is dependent upon, at least in part, the method used to form the gate insulator layer 526. For example, a thermal process generally results in an embodiment similar to that illustrated in FIG. 5, while the gate insulator layer 526 may extend over the first dielectric material 320 when the gate insulator layer 526 is formed using, e.g., a CVD process or an ISSG process.
Optionally, an implant may be performed to aid or retard the formation of the gate insulator layer 526. For example, a nitrogen implant may be performed to retard an oxide growth in select areas, such as the bottom of the trench, and a fluorine implant may be performed to increase the oxide growth. In an embodiment, a nitrogen implant may be performed at an angle normal to the upper surface of the substrate. In this embodiment, sidewalls of the trenches will be implanted less than the bottom surface of the trench. The nitrogen implant along the bottom of the trench retards the oxide growth, thereby resulting in a thinner gate insulator layer along the bottom of the trenches as compared to the sidewalls of the trench. In another embodiment, the implant angle may be adjusted to implant nitrogen along the sidewalls, thereby resulting in a thicker gate insulator along the bottom as compared to the sidewalls. Similar effects, e.g., relatively thinner or thicker gate insulator layer along the bottom of the trenches, may be obtained using a fluorine implant to increase the relative growth rate of the gate insulator layer.
It should be noted that the substrate 110 may be doped before forming the gate insulator layer to prepare, for example, the channel region. For example, in forming a p-type transistor having p-type doped source/drain regions, an n-type dopant, such as phosphorous, arsenic, nitrogen, antimony, or the like, may be implanted into the channel region (along the sidewalls and bottom of the trenches 2162 and 2164) of the substrate 110 prior to forming the gate insulator layer 526. Similarly, in forming an n-type transistor having n-type doped source/drain regions, a p-type dopant, such as boron, aluminum, gallium, indium, or the like, may be implanted into the channel region of the substrate. The implant angle may be adjusted to ensure proper implantation along the sidewall regions of the trenches 2162 and 2164 as well as the bottom of the trenches 2162 and 2164. Alternatively, the substrate 110 may be doped prior to forming the trenches by forming an n-well or a p-well, respectively, in which the trenches 2162 and 2164 are formed.
For example, a p-type transistor may be formed by implanting phosphorous ions at an angle of about 0° to about 5° relative to the bottom surface of the trenches 2162 and 2164 and at an angle between about −25° to about 25° relative to of a vertical sidewall of the trenches 2162 and 2164 at a dose of about 1E12 to about 3E13 atoms/cm2 and at an energy of about 20 to about 400 KeV. An n-type transistor may be formed by implanting boron ions at an angle of about 0° to about 5° relative to the bottom surface of the trenches 2162 and 2164 and at an angle between about −25° to about 25° relative to a vertical sidewall of the trenches 2162 and 2164 at a dose of about 1E12 to about 3E13 atoms/cm2 and at an energy of about 5 to about 300 KeV.
The gate electrode material 528 comprises a conductive material, such as a metal (e.g., tantalum, titanium, molybdenum, tungsten, platinum, aluminum, hafnium, ruthenium), a metal silicide (e.g., titanium silicide, cobalt silicide, nickel silicide, tantalum silicide), a metal nitride (e.g., titanium nitride, tantalum nitride), doped poly-crystalline silicon, other conductive materials, or a combination thereof. In one example, amorphous silicon is deposited and recrystallized to create poly-crystalline silicon (poly-silicon). In an embodiment, the gate electrode layer is formed by depositing, e.g., CVD, low-pressure CVD (LPCVD), and the like, a conformal layer covering the substrate 110 and filling the trenches 2162 and 2164. Thereafter, a planarizing process, such as a CMP process, may be performed to remove excess material, thereby forming a structure similar to that illustrated in FIG. 5.
The gate electrode material 528 may be deposited doped or undoped. For example, in an embodiment the gate electrode material 528 may be formed by depositing a polysilicon layer and, once applied, the polysilicon may be doped with, for example, phosphorous ions (or other P-type dopants) to form a PMOS device or boron (or other N-type dopants) to form an NMOS device. The polysilicon may also be deposited, for example, by furnace deposition of an in-situ doped polysilicon. Alternatively, the gate electrode material 528 may comprise a polysilicon metal alloy or a metal gate comprising metals such as tungsten, nickel, titanium, and titanium nitride, and the like, for example.
FIG. 6 illustrates recessing of the gate electrode material 528 (see FIG. 5) to form the gate electrodes 630 along the bottom portions of the trenches 2162 and 2164. In embodiments in which the gate electrode material 528 comprises polysilicon, the recessing may be performed using either dry or wet etching. In the case dry etching is used, the process gas may include CF4, CHF3, NF3, SF6, Br2, HBr, Cl2, or combinations thereof. Diluting gases such as N2, O2, or Ar may optionally be used. In the case wet etching is used, the chemicals may include NH4OH:H2O2:H2O (APM), NH2OH, KOH, HNO3:NH4F:H2O, and/or the like. In an embodiment, the gate electrode material 528 is recessed from about 500 Å to about 2,000 Å.
Referring now to FIG. 7, a second dielectric layer 732 is formed over the substrate 110, filling the recesses above the gate electrodes 630 in the trenches 2162 and 2164. The second dielectric layer 732 may be formed of similar materials using similar processes as those discussed above with reference to the first dielectric material 320. After depositing the second dielectric layer 732, a planarization process, e.g., a CMP process, may be used to remove excess material, thereby forming a structure similar to that illustrated in FIG. 7. In an embodiment, this planarization process exposes the fins 218.
FIG. 8 illustrates formation of the source/drain regions 834 in accordance with an embodiment. The source/drain regions 834 may be doped by implanting n-type or p-type dopants. For example, n-type transistor may be formed by implanting an n-type ion such as phosphorous ions, at a dose of about 1E15 to about 5E15 atoms/cm2 and at an energy of about 20 to about 100 KeV. A p-type transistor may be formed by p-type ions, such as boron ions, at a dose of about 1E15 to about 5E15 atoms/cm2 and at an energy of about 10 to about 50 KeV.
Furthermore, FIG. 8 also illustrates optional silicide areas 836 in accordance with an embodiment. The silicide areas 836 reduce contact resistance between the source/drain regions 834 and contacts formed in subsequent processing steps. The silicide areas 836 may be formed, for example, by depositing a metal layer (not shown) such as titanium, nickel, tungsten, or cobalt via plasma vapor deposition (PVD) procedures. An anneal procedure causes the metal layer to react with the substrate 110, e.g., silicon, of the source/drain regions 834 to form metal silicide. Portions of the metal layer overlying other areas, such as the first dielectric material 320 (e.g., the isolation structures) and second dielectric layer 732 remain unreacted. Selective removal of the unreacted portions of the metal layer may be accomplished, for example, via wet etch procedures. An additional anneal cycle may be used if desired to alter the phase of silicide areas 836, which may result in a lower resistance.
As can be appreciated, the above paragraphs describe embodiments of an embedded transistor that may be used in a variety of applications. For example, FIGS. 9, 10A, and 10B illustrate an embodiment in which the embedded transistor disclosed above is utilized as an access transistor in a DRAM memory cell. In particular, FIG. 9 illustrates a plan view of a plurality of DRAM memory cells, FIG. 10A illustrates a cross-sectional view along the A-A′ line of FIG. 9, and FIG. 10B illustrates a cross-sectional view along the B-B′ line of FIG. 9. A single memory cell is designated by the dashed box 950.
The memory cell 950 includes bitline 952 formed in, for example, the first metallization layer M1 having bitline contacts 954 electrically coupling the bitline 952 to one of the source/drain regions 834 of the underlying access transistor. The other of the source/drain regions 834 of the access transistor is electrically coupled to a storage node 956 via storage node contacts 958. The storage node 956 may be, for example, a metal-insulator-metal (MIM) capacitor, a planar capacitor, a U-shaped capacitor, a vertical capacitor, a horizontal capacitor, a non-capacitor storage structure, or the like. The gate electrode 630 is electrically coupled to a wordline (WL).
It should be appreciated that embodiments such as some of those discussed above utilize a single mask and etch process to form the isolation trenches and the embedded gate electrodes. In this manner, embodiments disclosed herein using a self-aligned process avoid misalignment issues that may be seen in other approaches in which the isolation trenches and the gate electrode trenches are formed with separate mask and etching processes. It is believed that these embodiments reduce wordline disturbance issues.
Embodiments disclosed herein also allow layout designers greater freedom. For example, the gate length is defined by the depth of the trench as opposed to the pitch between fins, thereby possibly allowing the gate length to be adjusted without increasing the pitch.
In an embodiment, a method is provided. The method comprises providing a substrate having a plurality of trenches; forming a dielectric material in first ones of the plurality of trenches; forming a gate dielectric along sidewalls and a bottom of second ones of the plurality of trenches; forming a gate electrode over the gate dielectric in the second ones of the plurality of trenches; and forming source/drain regions in the substrate on opposing sides of the second ones of the plurality of trenches.
In yet another embodiment, another method is provided. The method comprises providing a substrate; forming a plurality of trenches in the substrate; filling the plurality of trenches in the substrate with a first dielectric material; removing at least a portion of the first dielectric material from select ones of the plurality of trenches; forming a gate dielectric layer along a bottom and sidewalls of the select ones of the plurality of trenches; forming a gate electrode over the gate dielectric layer along a bottom of the select ones of the plurality of trenches; and forming source/drain regions along a surface of the substrate adjacent the select ones of the trenches.
In still yet another embodiment, a device is provided. The device comprises a substrate; a first recess and a second recess in the substrate, the first recess and the second recess having a first depth; a dielectric material in the first recess; a gate dielectric along sidewalls and a bottom of the second recess; a gate electrode over the gate dielectric, the gate electrode being recessed below an uppermost surface of the substrate; and source/drain regions in the substrate on opposing sides of the second recess.
In yet still another embodiment, a device is provided. The device includes a substrate and a shallow trench isolation in the substrate. A recess is in the substrate, wherein the recess has a same depth as the shallow trench isolation. A gate dielectric is a bottom and sidewalls of the recess, and a gate electrode is over the gate dielectric, wherein the gate dielectric completely separates the gate electrode from the substrate. The device further includes a dielectric layer over the gate electrode, wherein the dielectric layer extends into the recess, and source/drain regions in the substrate on opposing sides of the recess.
In yet still another embodiment, a device is provided. The device includes a substrate and a shallow trench isolation in the substrate. A first recess is formed in the substrate, wherein the first recess having a same depth as the shallow trench isolation. A gate dielectric is along a bottom and sidewalls of the recess, and a gate electrode is over the gate dielectric. A first source/drain region is along a surface of the substrate on a first side of the first recess, and a second source/drain region is along the surface of the substrate on a second side of the first recess. A first contact is electrically coupled between the first source/drain region and a bit line, and a second contact is electrically coupled between the second source/drain region and a storage node, wherein the first contact and the second contact are laterally offset along a longitudinal access of the first recess.
Although the present disclosure and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the disclosure as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, and composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the present disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present disclosure. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.

## Claims

1. A device comprising:
a substrate;
a first recess and a second recess in the substrate, the first recess having a first bottom and the second recess having a second bottom, the first bottom and the second bottom being at a same first depth;
a dielectric material filling the first recess;
a gate dielectric along sidewalls and a bottom of the second recess;
a gate electrode over the gate dielectric, the gate electrode having a top surface below an uppermost surface of the substrate; and
source/drain regions in the substrate on opposing sides of the second recess.

2. The device of claim 1, further comprising a storage node electrically coupled to one of the source/drain regions.

3. The device of claim 2, wherein the storage node comprises a metal-insulator-metal (MIM) capacitor.

4. The device of claim 2, further comprising a bit line electrically coupled to one of the source/drain regions.

5. The device of claim 1, wherein the gate electrode is electrically coupled to a word line.

6. The device of claim 1, wherein a width of the first recess is different than a width of the second recess.

7. The device of claim 1, wherein the first same depth is between about 1,000 Å and about 4,000 Å.

8. A device comprising:
a substrate;
a shallow trench isolation in the substrate, the shallow trench isolation being filled with a dielectric material;
a recess in the substrate, a bottom of the recess having a same depth as a bottom of the shallow trench isolation;
a gate dielectric along a bottom and sidewalls of the recess;
a gate electrode over the gate dielectric, the gate dielectric completely separating the gate electrode from the substrate;
a dielectric layer over the gate electrode, the dielectric layer extending into the recess; and
source/drain regions in the substrate on opposing sides of the recess.

9. The device of claim 8, wherein the gate dielectric extends to an uppermost surface of the substrate.

10. The device of claim 8, wherein an uppermost surface of the dielectric layer is coplanar with an uppermost surface of the shallow trench isolation.

11. The device of claim 8, wherein the shallow trench isolation has a first width and the recess has a second width, the first width being different than the second width.

12. The device of claim 8, wherein the gate dielectric has a different thickness along a bottom than along sidewalls of the recess.

13. The device of claim 8, further comprising a metal-insulator-metal (MIM) capacitor coupled directly to one of the source/drain regions.

14. The device of claim 13, further comprising a bit line electrically coupled to another one of the source/drain regions.

15. The device of claim 14, wherein the gate electrode is electrically coupled to a word line.

16. A device comprising:
a substrate;
a shallow trench isolation in the substrate, the shallow trench isolation being completely filled with dielectric material;
a first recess in the substrate, a depth of a bottom of the first recess having a same depth as a bottom of the shallow trench isolation;
a gate dielectric along a bottom and sidewalls of the first recess;
a gate electrode over the gate dielectric;
a first source/drain region along a surface of the substrate on a first side of the first recess;
a second source/drain region along the surface of the substrate on a second side of the first recess;
a first contact electrically coupled between the first source/drain region and a bit line; and
a second contact electrically coupled between the second source/drain region and a storage node, wherein the first contact and the second contact are laterally offset along a longitudinal access axis of the first recess.

17. The device of claim 16, further comprising a second recess in the substrate, wherein the shallow trench isolation is interposed between the first recess and the second recess.

18. The device of claim 16, further comprising a dielectric layer over the gate electrode, an uppermost surface of the dielectric layer being coplanar with an uppermost surface of the substrate.

19. The device of claim 16, wherein the shallow trench isolation has a first width and the first recess has a second width, the first width being different than the second width.

20. The device of claim 16, wherein the gate dielectric has a first thickness along a bottom of the first recess and a second thickness along a sidewall of the first recess, wherein the first thickness is different than the second thickness.