# Patent

## Title
DRAM including a vertical surround gate transistor

## Abstract

DRAM memory cells having a feature size of less than about 4F2 include vertical surround gate transistors that are configured to reduce any short channel effect on the reduced size memory cells. In addition, the memory cells may advantageously include reduced resistance word line contacts and reduced resistance bit line contacts, which may increase a speed of the memory device due to the reduced resistance of the word line and bit line contacts.

## Background
RELATED APPLICATION
This application is a continuation of U.S. application Ser. No. 11/188,507, filed Jul. 25, 2005, which is hereby incorporated by reference in its entirety herein.


BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to microelectronic devices and related fabrication methods. More particularly, the invention relates to microelectronic vertical field effect transistors and related fabrication methods.
2. Description of the Related Art
Since the introduction of the digital computer, electronic storage devices have been a vital resource for the retention of data. Conventional semiconductor electronic storage devices, such as Dynamic Random Access Memory (DRAM), typically incorporate capacitor and transistor structures in which the capacitors temporarily store data based on the charged state of the capacitor structure. In general, this type of semiconductor Random Access Memory (RAM) often requires densely packed capacitor structures that are easily accessible for electrical interconnection.
A dynamic random access memory cell typically comprises a charge storage capacitor (or cell capacitor) coupled to an access device, such as a Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET, or simply FET). These access devices function to apply or remove charge on the capacitor, thus affecting a logical state defined by the stored charge. The amount of charge stored on the capacitor is determined by the electrode (or storage node) area and the interelectrode spacing. The conditions of DRAM operation such as operating voltage, leakage rate and refresh rate, will generally mandate that a certain minimum charge be stored by the capacitor.
FETs are widely used in integrated circuit devices including logic, memory and/or microprocessor devices that are used in consumer and/or industrial applications. For example, FETs are commonly used as the access device for DRAM memories. As the integration density of integrated circuit FETs continues to increase, it may be desirable to continue to shrink the dimensions of the FETs. Conventionally, features of integrated circuit FETs may be formed on a microelectronic substrate, such as silicon semiconductor substrate, using photolithography and etching. Unfortunately, as the minimum feature size scales into the sub-0.1 micron region, it may be increasingly difficult to define such small features using traditional lithography and etching. Although improved nano-lithography techniques may be developed, it still may be difficult to reliably define features as small as 35 nm or smaller in a controllable and cost-effective way using lithography, to allow mass production.
In order to increase efficiency of memory devices, there is a similar effort to create smaller memory cells. DRAM memory cells can shrink in several ways. One way to decrease the size of a memory cell is to reduce the minimum feature size (F). This generally occurs through new and advanced lithography and etching techniques. Memory cells can also be decreased by designing a smaller memory cell. For example many of the DRAM chips on the market today have a memory cell size of 8F2 or greater, where F is the dimension of the minimum feature for a given manufacturing process. However, as the size of FETs and memory cells continue to decrease, there is an increase in the electrostatic charge sharing between gate and source-drain regions of the transistor devices. This electrostatic charge sharing is typically referred to as the short channel effect. As those of skill in the art readily recognize, as the length of the transistor channel decreases, the threshold voltage of the transistor also increases due to the short channel effect. Thus, there is a need for improved systems and methods of reducing the size of memory devices, while reducing the short channel effect on the reduced size memory devices.
SUMMARY OF THE INVENTION
Processes for forming memory cells including vertical surround gate transistors are disclosed. In an advantageous embodiment, the memory cells have a feature size of less than about 4F2. In one embodiment, a 4F2 DRAM comprises a vertical surround gate transistor.
In one embodiment, a DRAM memory device comprises a vertical transistor comprising a source, a drain, a surround gate, and a channel region. The DRAM memory device further comprises a bit line electrically coupled to the drain of the vertical transistor, wherein the gate comprises a word line of the memory device, and a capacitor electrically coupled to the source.
In one embodiment, a method of manufacturing a DRAM memory device having a feature size of less than about 4F2 comprises forming a vertical surround gate transistor comprising a source, a drain, a surround gate, and a channel region, wherein, the gate comprises a word line of the memory device. The method further comprises forming a bit line so that the bit line is electrically coupled to the drain of the vertical transistor, and forming a capacitor so that the capacitor is electrically coupled to the source.
In another embodiment, a DRAM memory device comprises a vertical transistor comprising a source, a drain, a gate, and a channel region, wherein, at least a portion of the gate is silicided to form a word line contact of the memory device. The DRAM memory device further comprise a bit line electrically coupled to the drain of the vertical transistor, and a capacitor electrically coupled to the source.
In another embodiment, a method of forming a memory device having a vertical surround gate transistor comprising forming a semiconductor substrate comprising a first layer having a first doping and a second layer above the first layer having a doping opposite the first doping, forming a silicided drain contact in electrical contact with the second layer, forming a dielectric layer on a portion of the silicided drain contact, forming a vertically extending polysilicon gate on the dielectric layer, forming a vertically extending silicided gate on the dielectric layer, epitaxially growing a channel region on the second layer so that the polysilicon gate is sandwiched between the channel region and the silicided gate, epitaxially growing a source region on the channel region so that a portion of the source region is in electrical contact with the polysilicon gate, and forming a capacitor in electrical contact with the source region.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A is a schematic top view of a portion of a memory device;
FIG. 1B is a schematic top view of a portion of a memory device;
FIG. 2 is a diagrammatic section view of the memory device illustrated in FIG. 1, taken along line A-A′.
FIG. 3 is a view of FIG. 2 shown at a processing stage subsequent to that of FIG. 2.
FIG. 4 is a view of FIG. 3 shown at a processing stage subsequent to that of FIG. 3.
FIG. 5 is a view of FIG. 4 shown at a processing stage subsequent to that of FIG. 4.
FIG. 6 is a view of FIG. 5 shown at a processing stage subsequent to that of FIG. 5.
FIG. 7 is a view of FIG. 6 shown at a processing stage subsequent to that of FIG. 6.
FIG. 8 is a view of FIG. 7 shown at a processing stage subsequent to that of FIG. 7.
FIG. 9 is a view of FIG. 8 shown at a processing stage subsequent to that of FIG. 8.
FIG. 10 is a view of FIG. 9 shown at a processing stage subsequent to that of FIG. 9.
FIG. 11 is a view of FIG. 10 shown at a processing stage subsequent to that of FIG. 10.
FIG. 12 is a view of FIG. 11 shown at a processing stage subsequent to that of FIG. 11.
FIG. 13 is a view of FIG. 12 shown at a processing stage subsequent to that of FIG. 12.
FIG. 14 is a view of FIG. 13 shown at a processing stage subsequent to that of FIG. 13.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Embodiments of the invention will now be described with reference to the accompanying Figures, wherein like numerals refer to like elements throughout. The terminology used in the description presented herein is not intended to be interpreted in any limited or restrictive manner, simply because it is being utilized in conjunction with a detailed description of certain specific embodiments of the invention. Furthermore, embodiments of the invention may include several novel features, no single one of which is solely responsible for its desirable attributes or which is essential to practicing the inventions herein described.
In the context of this document, the term “semiconductor substrate” is defined to mean any construction comprising semiconductor materials, including, but not limited to, bulk semiconductor materials such as a semiconductor wafers, and semiconductor material layers. The term “substrate” refers to any supporting substrate, including, but not limited to, the semiconductor substrates (either alone or in assemblies comprising other materials thereon) described above. Also in the context of this document, the term “layer” encompasses both the singular and the plural unless otherwise indicated.
Double gate and/or surround gate FETs have been proposed to reduce the short channel effect. A double/surround gate FET may include a thin channel that is controlled by both a front gate and a back gate. Short channel effects may be suppressed because the two gates can be effective in terminating drain field lines and preventing the drain potential from impacting the source. Double gate devices may be extended to provide surround gate devices in which the gate wraps around the channel. FETs including double/surround gate FETs may be grouped into two categories based on the channel orientation. In horizontal devices, carrier conduction from source to drain through the channel occurs in a direction that is generally parallel to the face of the microelectronic substrate. In contrast, in vertical devices, carrier conduction from source to drain through the channel occurs in the vertical direction, generally orthogonal to the face of the microelectronic substrate.
Vertical transistor designs can be used to decrease chip real estate occupied by a memory cell transistor. An example of a memory cell with a vertical transistor is disclosed in U.S. Pat. No. 6,756,625, issued to Brown, the disclosure of which is incorporate by reference herein.
The following description describes memory device structures that advantageously have a smaller feature size than is currently known in the art and reduce the short channel effect on the memory device. In an advantageous embodiment, the memory devices have a feature size of about 4F2. In other embodiments, memory devices having features sizes of less than 4F2 may also be manufactured according to the methods described herein. In addition, embodiments of memory devices having low resistance word lines and/or bit lines, which may allow the memory devices to operate at higher frequencies, are also described. Methods of fabricating these memory devices are also disclosed herein.
FIG. 1A is a schematic top view of a portion of a memory device 100. As illustrated in FIG. 1A, the memory device 100 comprises word lines 110 and bit lines 120. In a memory device, such as DRAM, each of the memory cells includes a capacitor 130 and an epitaxially grown pillar 140 that consists of the source, drain, gate and channel region of the memory cell. In one embodiment, the word lines 110 and bit lines 120 are non-orthogonal. For example, FIG. 1B is a top view of a memory cell comprising non-orthogonal word lines 110 and bit lines 120, wherein the memory cell has a feature size of about 4F2.
FIGS. 2-14 are each diagrammatic section views of the memory device illustrated in FIG. 1. These figures illustrate an exemplary process of fabricating a memory device that includes a surround gate vertical transistor and advantageously has a feature size of about 4F2. In addition, embodiments of the memory device also include low resistance word line and bit line contacts, and combine a vertical transistor with a stack capacitor. The following example is provided as an illustration of one method of forming a memory device according to the general systems and methods described herein. Accordingly, the invention is not limited to the specific embodiments described with respect to FIGS. 2-14. In particular, other embodiments of memory devices having one or more of the features described with reference to the memory device illustrated in FIGS. 2-14 are contemplated.
FIG. 2 is a diagrammatic section view of the memory device illustrated in FIG. 1. In FIG. 2, the memory device 100 is at an initial stage of fabrication. A shallow trench isolation (STI) 220 area has been etched into a semiconductor wafer 210. In the embodiment of FIG. 2, the semiconductor wafer 210 includes two layers 210A and 210B that are doped with oppositely charged ions. For example, in one embodiment the semiconductor layer 210A is an N-type semiconductor material while the semiconductor layer 210B is a P-type semiconductor material. However, in other embodiments, the doping of the semiconductor wafer 210 may be patterned differently. For example, in one embodiment the semiconductor layer 210A may be P-type and the semiconductor layer 210B may be N-type. In one embodiment, the semiconductor wafer 210A is about 750 Angstroms thick. In one embodiment, the STI 220 is about 2,000 Angstroms deep in the semiconductor wafer 210. In one embodiment, the STI 220 is filled with an oxide, such as may be formed using a High Density Plasma (HDP) Chemical Vapor Deposition (CVD) process.
With the semiconductor wafer 210 patterned with the STI 220, an oxide layer 230 is deposited on the semiconductor wafer 210. In one embodiment, the oxide layer is about 500 Angstroms thick and is deposited using a CVD process. Next, a nitride layer 240 is deposited on the oxide layer 230 using a CVD process, for example. In one embodiment, the nitride layer 240 is about 200 angstroms thick. Finally, a thick oxide layer is deposited on the surface of the nitride layer 240, and is patterned and etched using a Reaction Ion Etch (RIE) process, for example, to form pillars 250. In the embodiment of FIG. 2, the RIE process stops etching at the top surface of the nitride layer 240. In one embodiment, the oxide pillars 250 are about 3000 angstroms thick. In other embodiment, the thickness of these layers may be adjusted in order to achieve varied results.
Moving to FIG. 3, the memory device 100 is further processed. In particular, dielectric spacers 310 are formed on the lateral edges of the pillars 250 by dielectric deposition and an anisotropic RIE. The nitride layer 240 and the oxide layer 230 are then selectively etched, stopping at the semiconductor layer 210A. In one embodiment, the dielectric spacers 310 comprise nitride materials, such as Silicon Nitride. In one embodiment, the spacers 310 are about 200 Angstroms thick.
Turning to FIG. 4, additional doped layers of semiconductor material are grown between the pillars 250. In one embodiment, layer 210A is epitaxially extended, with the same doping as originally used in layer 210A of FIGS. 1 and 2, so that layer 210A extends along layer 230 and optionally up to or past layer 240. Layers 410 and 420 are also epitaxially grown between the spacers 310 that surround lateral sides of the pillars 250. In the exemplary embodiment of FIG. 4, doped layer 210A is epitaxially thickened and then the semiconductor layer 410 (which is doped with the same type of doping, e.g., N or P type doping, as semiconductor layer 210B) is grown between the spacers 310, followed by growing of the semiconductor layer 420 (which is doped with the same type of doping as semiconductor layer 210A). Thus, the entire stack of semiconductor material now comprises alternatively doped layers 210B, 210A, 410, and 420.
Moving to FIG. 5, the dielectric spacer 310 is removed, such as by using a chemical etching process selective to the dielectric material of the oxide layer 230 and the pillars 250, leaving a void 312. In FIG. 6, a thin gate oxide (not shown) is grown on the memory device 100. More particularly, the thin gate oxide is grown on the exposed surfaces of semiconductor layers 210A, 410, 420. Subsequently, a polysilicon layer 511 is deposited on the exposed surfaces of the memory device 100. As illustrated in FIG. 6, the polysilicon layer 511 covers the pillars 250.
In FIG. 7, a portion of the polysilicon 511 is etched back using either an RIE or chemical etching process selective to the thin gate oxide. This etching exposes an upper portion of the pillars 250 and a portion of the semiconductor layer 420, leaving a void 512. In one embodiment, the polysilicon 511 is removed to an elevational level that is above the semiconductor layer 410. In FIG. 8, a dielectric material 810 is deposited in the void 512 (FIG. 7). In one embodiment, the dielectric 810 is a nitride, such as Silicon Nitride, for example. However, the dielectric 810 may comprise any other dielectric, or combinations of dielectrics. In one embodiment, the dielectric 810 is planarized using Chemical Mechanical Polishing (CMP), for example so that an upper surface of the dielectric 810 is aligned with the upper surface of the pillars 250.
In FIG. 9, the pillars 250 (FIGS. 2-8), which may comprise an oxide, are stripped away using a chemical process, for example, thereby forming trenches 910 between the polysilicon 511 and the dielectric 810. In one embodiment, a RIE process is used to remove portions of the nitride layer 240 (e.g., FIG. 8) and oxide 230 (e.g., FIG. 8). As illustrated in FIG. 9, a first trench 910A exposes the semiconductor layer 210A.
In FIG. 10, a thin metal 1010 is deposited on the exposed surfaces of the memory device 100. In one embodiment, the metal comprises cobalt or nickel. In one embodiment, the thin metal 1010 is covered with a Ti or TiN layer. In one embodiment, the exposed thin metal 1010 is exposed to an increased temperature that is sufficiently high to react the Cobalt or Nickel portions with the polysilicon layer 511. This reaction forms a silicided layer 1110 (e.g., FIG. 11). The non-reacting portions of the thin metal 1110, such as above the silicided layer 1110, may then be stripped using a chemical etch, for example. In one embodiment, portions of the metal will be used as bit and word line contacts of the memory device 100.
Moving to FIG. 11, with a portion of the polysilicon layer 511 silicided, the gate of the vertical transistor becomes a surround gate structure, including a silicided gate 1110 and a poly silicon gate 511. Because a surround gate structure is used in the memory device 100, the short channel effects within the memory device are advantageously reduced. In addition, due to the silicidation of the gate contact 1110, a low resistance word line is formed. Similarly, due to the silicidation of the semiconductor layer 210A, a low resistance drain contact 1120 is formed. As those of skill in the art will appreciate, as the resistance of the bitline and wordline of a memory device are decreased, the operating frequency of the memory device may be correspondingly increased. Accordingly, in one embodiment the silicidation of the word line and bit line provides a lower resistance, faster, memory device.
In FIG. 12, a nitride spacer 1210 is formed to cover the sidewalls of the trenches 910 (e.g., FIG. 9), including the gate contact 1110. In one embodiment, a nitride film is deposited by CVD and an anisotropic etch is used to form the nitride spacer 1210. Next, a dielectric 1220 is deposited using a CVD process, for example, between the nitride spacers 1210. In one embodiment, the dielectric 1220 is oxide and is planarized using a process such as CMP.
In FIG. 13, a bit line to drain contact 1310 is created (see FIG. 1A also). In one embodiment, the path 1310 comprises Tungsten. In one embodiment, the drain contact 1310 is formed by a mask that exposes dielectric 1220, performing a RIE to remove the dielectric 1220 selective to 1210, and depositing the drain contact material, such as Tungsten, followed by a CMP of the Tungsten.
In FIG. 14, the bit line 120 is formed on the contact 1310. In one embodiment, the bit line contact 120 comprises W, Al, Cu, or a combination of these metals. In the embodiment of FIG. 14, a dielectric 1420 is formed around the bit line 120. In one embodiment, the dielectric 1420 protects the bit line 120 from processes that may be used in formation of the capacitor 130. In another embodiment, the capacitor 130 may be formed prior to formation of the bit line 120 and the dielectric 1420 may be unnecessary.
Having completed the processing steps depicted in FIGS. 2-14, the resulting DRAM memory cell 1400 advantageously includes a reduced resistance word line, a reduced resistance bit line contact, and a surround gate vertical transistor. In addition, using the processing steps described above, or similar processes known in the art, the feature size of the memory cells may be reduced without increasing the short channel effect. In an advantageous embodiment, the memory cell 115 (FIG. 1) has a feature size of about 4F2.
The foregoing description details certain embodiments of the invention. It will be appreciated, however, that no matter how detailed the foregoing appears in text, the invention can be practiced in many ways. As is also stated above, it should be noted that the use of particular terminology when describing certain features or aspects of the invention should not be taken to imply that the terminology is being re-defined herein to be restricted to including any specific characteristics of the features or aspects of the invention with which that terminology is associated. The scope of the invention should therefore be construed in accordance with the appended claims and any equivalents thereof.

## Claims

1. A method of forming a vertical memory cell, the method comprising:
forming a vertical memory cell comprising:
a source,
a drain,
a polysilicon surround gate to extend in a first generally vertically extending direction, and
a generally vertically extending channel region, wherein the source is arranged at an upper end of the vertically extending channel region and wherein the drain is arranged at an opposite lower end of the vertically extending channel region;

depositing a dielectric to cover a horizontal upper surface of the polysilicon surround gate;
forming a trench extending along the surround gate so as to expose a vertical surface of the surround gate;
depositing a thin metal on the exposed vertical surface of at least the polysilicon surround gate while the horizontal upper surface of the polysilicon surround gate remains covered by the dielectric;
exposing the thin metal to heat that is sufficiently high to react at least a portion of the thin metal with at least a portion of the polysilicon gate in order to form a silicided gate extending along a vertical length of the formerly exposed vertical surface of the polysilicon surround gate;
forming a spacer arranged to cover at least the silicided gate contact; and
forming a second separate generally vertical conductive path extending from a drain contact upwards to above the surround gate, wherein the drain contact is arranged adjacent the lower end of the vertically extending channel region.

2. The method of claim 1, wherein the thin metal comprises Cobalt.

3. The method of claim 1, wherein the thin metal comprises Nickel.

4. The method of claim 1, wherein the thin metal comprises Titanium.

5. The method of claim 1, wherein the thin metal comprises a layer of either Cobalt or Nickel and a layer of either Titanium or Titanium Nitride.

6. The method of claim 1, further comprising siliciding at least a portion of the drain such that the drain contact is at least partially silicided.

7. The method of claim 1, wherein the polysilicon surround gate comprises a word line contact.

8. The method of claim 1, wherein the memory cell comprises a DRAM cell.

9. The method of claim 1, further comprising forming a capacitor in contact with the source.

10. A vertical memory cell comprising:
a source region;
a drain region;
a polysilicon surround gate extending generally vertically between the source region and drain region;
a channel region extending generally vertically between the source region and the drain region wherein the source region is arranged at an upper end of the vertically extending channel region and wherein the drain region is arranged at an opposite lower end of the vertically extending channel region;
a vertically extending silicided surround gate contact arranged adjacent the polysilicon surround gate wherein the polysilicon surround gate and the silicided surround gate contact extend along a vertical length of the channel region;
an insulating sidewall spacer arranged to cover a vertical outer surface of the silicided gate contact; and
a separate second conductive path extending generally vertically from a silicided drain contact over the drain region to above the polysilicon surround gates and the spacer and wherein the drain contact is arranged adjacent the lower end of the vertically extending channel region, the second conductive path insulated from the silicided surround gate contact by the insulating sidewall spacer.

11. A method of forming a memory device comprising a plurality of vertical memory cells, the method comprising:
forming a plurality of vertical memory cells each comprising a polysilicon surround gate vertically extending over a channel region, a source arranged generally at an upper end of the channel region, and a drain arranged generally at an opposite lower end of the channel region;
depositing dielectric to cover upper surfaces of the polysilicon surround gates;
forming trenches to expose vertical surfaces of the polysilicon surround gates;
depositing a thin metal on the exposed vertical surfaces of the polysilicon surround gate of each memory cell;
exposing the thin metal to heat that is sufficiently high to react at least a portion of the thin metal with at least a portion of the polysilicon gate in order to form a silicided gate contact adjacent and extending across an entire vertical length of the polysilicon surround gate in each of the memory cells;
forming sidewall spacers arranged to cover at least the silicided gate contacts; and
forming second separate generally vertical conductive paths extending from respective drain contacts upwards to above the respective surround gate, wherein the drain contacts are arranged adjacent the lower end of the respective vertically extending channel region and wherein the second generally vertical conductive paths are isolated from the silicided gate contacts by the sidewall spacers.

12. The method of claim 11, further comprising siliciding at least a portion of the drains of each of the memory cells such that the drain contacts in each memory cell are at least partially silicided.

13. The method of claim 12, wherein bit lines of the memory device are electrically connected to first subsets of the silicided drain contacts via respective separate second vertical conductive paths.

14. The method of claim 11, wherein a word line of the memory device comprises a second subset of the polysilicon surround gates.

15. The method of claim 11, wherein the thin metal comprises material selected from the group comprising Tungsten, Nickel, and Cobalt.

16. The method of claim 11, further comprising depositing a second thin metal on the thin metal prior to exposing the thin metal to heat.

17. The method of claim 16, wherein the second thin metal comprises material selected from the group comprising Titanium and Titanium Nitride.

18. The method of claim 11, wherein the memory device comprises a DRAM.

19. The method of claim 1, further comprising forming a bit line in contact with the separate second generally vertical conductive path such that the bit line is arranged at an elevation above the surround gate.

20. The method of claim 19, further comprising forming a word line such that the word line and the bit line are non-orthogonal.

21. The method of claim 11, further comprising forming a plurality of word lines and a plurality of bit lines such that the word lines and the bit lines are non-orthogonal.

22. The method of claim 11, further comprising forming bit lines in contact with respective separate second generally vertical conductive paths such that the bit lines are arranged at an elevation above the respective surround gates.