Epitaxial monocrystalline channel for storage transistors in 3-dimensional memory structures and methods for formation thereof

Abstract

A thin-film storage transistor includes (a) first and second semiconductor regions comprising polysilicon of a first conductivity; and (b) a channel region between the first and second semiconductor regions, the channel region comprising single-crystal epitaxial grown silicon, and wherein the thin-film storage transistor is formed above a monocrystalline semiconductor substrate.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to memory structures, formed on top of a monocrystalline semiconductor substrate, that are organized as arrays of thin-film storage transistors. In particular, the present invention relates to providing a single-crystal silicon channel region for such storage transistors.

2. Discussion of the Related Art

The Copending Application discloses 3-dimensional memory structures, formed on top of a monocrystalline semiconductor substrates, that are organized as arrays of NOR memory strings. In this context, the term “NOR memory string” refers to a group of thin-film storage transistors sharing common source and drain regions. FIG. 1 illustrates, in cross section, memory structure 30 including an array of NOR memory strings. As shown in FIG. 1, memory structure 30 is formed on a semiconductor (e.g., epitaxial monocrystalline silicon) substrate 150. A suitable semiconductor substrate may be, for example, a semiconductor wafer that is used for fabrication of electronic circuits, as known to those of ordinary skill in the art.

Semiconductor substrate 150 may have fabricated thereon and therein various circuit elements (e.g., CMOS transistor 10) interconnected by conductors 22 (e.g., copper) in conventional interconnect layers through contacts or vias 16. These circuit elements are first fabricated on the semiconductor substrate using conventional techniques before forming memory structure 30. The interconnect layers—typically embedded in a dielectric layer—may include conductors intended for supporting operations of memory arrays in memory structure 30, which is to be formed over the interconnect layers. For example, interconnect layer 24 provides conductors (“global word lines 24”) that are intended to connect conductors 32 (e.g., heavily-doped polysilicon) serving as word lines that address storage transistors in memory structure 30. Conductors 32 are referred to as “local word lines” in this detailed description.

As shown in FIG. 1, memory structure 30 includes numerous stacks of “active strips” (e.g., active stacks 101a, 101b and 101c). For example, FIG. 1 shows stacks 101a, 101b and 101c each including 4 active strips isolated from each other by isolation layers 107. FIG. 1 is a cross section through the active strips, which extend lengthwise into and out of the plane of cross section. In this context, an active strip includes drain layer 104, source layer 103, body layer 102, and channel layers 108. (Channel layers 108 are provided on both sides of body layer 102.) In some implementations, drain layer 104 and source layer 103 are both n⁺ polysilicon, channel layers 108 are each p⁻ polysilicon and body layer 102 is a p⁺ polysilicon. In some implementations, rather than body layer 102, a dielectric layer may be used. Also shown in the active strips of FIG. 1 are conductive layers 105t and 105b, adjacent respectively to source layer 103 and drain layer 104, for reducing resistivity along the lengths of source layer 103 and drain layer 104. Along the each side of each slack of active strops (and, hence, along each side of each active strip) is provided a charge-trapping layer 107 and numerous local word lines 32. In FIG. 1, local words line 32 are conductive columns arranged along both sides of a stack of active strips. A storage transistor is constituted by a local word line, a portion of channel layer 108, the portion of charge-trapping layer 107 therebetween, and source and drain layers 103 and 104. Drain layer 104 and source layer 103 are shared by the numerous storage transistors formed along an active strip. Adjacent storage transistors along an active strip sharing common source and drain layers form a NOR memory string. (Turning on any storage transistor in the NOR memory string results in a conducting transistor current between the common source and drain layers.)

Unlike a channel region of a transistor formed in the monocrystalline semiconductor substrate, the polysilicon channel region of the thin-film storage transistors in memory structure 30 has several disadvantages:

• • (a) low conducting transistor current because of low electron mobility in the polysilicon;
  • (b) high temperature sensitivity in the transistor currents due to grain boundaries in the polysilicon;
  • (c) high dopant diffusivity in polysilicon, which requires careful design to mitigate out-diffusion of dopants into channel layers 108 from source and drain layers 103 and 104;
  • (d) high leakage currents in the bipolar junctions;
  • (e) polysilicon grain boundaries are deleterious to forming high-quality tunneling oxide in charge-trapping layer 107 difficult; and
  • (f) polysilicon grain boundaries introduce variability in the threshold voltages of the thin-film storage transistors.

Incorporating monocrystalline silicon in a memory structure for NAND type memory strings have been disclosed, for example, in the article, entitled “First Demonstration of Monocrystalline Silicon Macroni Channel for 3-D NAND Memory Devices,” published in Digest of Technical Papers, 2018 Symposium on VLSI Technology, pp. 203-204. The article discloses a process that first deposits a sacrificial amorphous silicon dummy channel, which is to be removed by chlorine gas in a carefully tuned isotropic etch prior to selective epitaxial silicon growth to form the ultimate channel. Removing the amorphous dummy channel must be carried out without significant undercut in features that provide mechanical integrity. Such precise tuning is particularly difficult in processes where features with high aspect ratios are present.

Processes for forming monocrystalline silicon features using selective silicon epitaxy are disclosed, for example, in (a) the article, entitled “Low Temperature Selective Silicon Epitaxy by Ultra-High Vacuum Rapid Thermal Chemical Vapor Deposition Using Si₂H₆, H₂ and Cl₂,” by K. Violette et al., Appl. Phys. Lett. 68, 66 (1996); (b) the Ph.D. dissertation, entitled “Silicon-Based Epitaxy by Chemical Vapor Deposition Using Novel Precursor Neopentasilane,” by Keith H. Chung, submitted in 2010 to Princeton University; and (c) the technical report, entitled “Modeling of Growth Rates of Selective Epitaxial Growth (SEG) and Epitaxial Lateral Overgrowth (ELO) of Silicon in the Si₂H₆-H₂—Cl₂ System,” by P. Kongretira et al., School of Electrical Engineering, Purdue University, October 1994.

SUMMARY

According to one embodiment of the present invention, a thin-film storage transistor includes (a) first and second semiconductor regions comprising polysilicon of a first conductivity; and (b) a channel region between the first and second semiconductor regions, the channel region comprising single-crystal epitaxial grown silicon, and wherein the thin-film storage transistor is formed above a semiconductor substrate.

According to one embodiment of the present invention, the thin-film storage transistors may be formed using a process which comprises: (i) providing a semiconductor substrate having a planar surface, the semiconductor substrate comprising monocrystalline silicon at the planar surface; (ii) providing an oxide isolation layer on top of the planar surface of the semiconductor substrate; (iii) forming a plurality of active layers, each active layer being isolated by a nitride isolation layer from its adjacent active layer or adjacent active layers, each active layer comprising (a) first and second semiconductor layers of a first conductivity; and (b) a dielectric layer between the first and second semiconductor layers; (iv) creating deep trenches in the plurality of active layers, to create a plurality of stacks of active strips separated by the deep trenches, each active strip being a portion of one of the active layer remaining in the stack of active strips as a result of creating the deep trenches; (v0 recessing the dielectric layer in each active strip to form one or more cavities in the active strip; (vi) in an evacuated reaction chamber, removing any native oxide on any exposed surface of the semiconductor substrate or the first and second semiconductor layers of the active strips; (vii) without removing the semiconductor substrate from the evacuated reaction chamber, filling the deep trenches and the cavities with single-crystal silicon using a selective epitaxial silicon growth technique; and (viii) removing the single-crystalline silicon from the deep trenches, thereby leaving single-crystalline silicon in the cavities to serve as channel regions of storage transistors to be subsequently formed.

The present invention is better understood upon consideration of the detailed description below in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates, in cross section, memory structure 30 including an array of NOR memory strings.

FIGS. 2(i), 2(ii), 2(iii), 2(iv), 2(v), 2(vi) and 2(vii) illustrate providing single-crystal silicon channel regions in storage transistors of a memory structure by selective epitaxial growth of silicon from a monocrystalline substrate, according to one embodiment of the present invention.

FIGS. 3(i), 3(ii), 3(iii), 3(iv), 3(v), 3(vi), 3(vii) and 3(viii) illustrate providing single-crystal silicon channels in a memory structure by selective epitaxial growth of silicon from either a crystalline substrate or polysilicon grain boundaries, according to alternative embodiments of the present invention.

To simplify the detailed description, like elements in the figures are assigned like reference numerals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides storage transistors with single-crystal semiconductor channel regions by either (a) epitaxial growth from an underlying semiconductor substrate or (b) initial epitaxial growth from the grain boundaries of N+ poly source and drain regions. This detailed description illustrates processes for forming such channel regions using memory structures such as those described above with respect to FIG. 1. However, for clarity of presentation, only one active strip is shown for each stack of active strips. One of ordinary skill in the art would understand that the processes illustrated herein are applicable for a stack of active strips with any suitable number of active strips.

FIGS. 2(i) to 2(vii) illustrate providing single-crystal silicon channel regions in storage transistors of a memory structure by selective epitaxial growth of silicon from a monocrystalline substrate, according to one embodiment of the present invention.

Initially, various circuit elements and interconnect layers are formed in isolation layer 151 on top of a planar surface of semiconductor substrate 150 (e.g. silicon), as described with respect to FIG. 1 above. Thereafter, multiple active layers are deposited on top of isolation layer 151. These active layers include nitride isolation layer 106, source layer 103, dielectric layer 102 (e.g., a silicon oxide layer), and drain layer 104, Again, for the purpose of clarity of presentation, some features not essential to the description below (e.g., conductive layers 105t and 105b) have been omitted from the figures. Stacks of active strips (e.g., those illustrated in FIG. 1 by stacks 101a, 101b and 101c) are then formed by etching deep trenches into the active layers (“NIN etch”). The resulting structure is shown in FIG. 2(i).

Thereafter, source layer 103 and drain layer 104, both consisting primarily of p⁺ polysilicon are recessed using a selective etch, which leaves dielectric layer 102 substantially intact. The resulting memory structure is illustrated by FIG. 2(ii). Dielectric layer 102 may also be recessed using a selective etch. The resulting memory structure is shown in FIG. 2(iii). A portion of isolation layer 151 is then removed to expose a portion of semiconductor substrate 150, which provides a monocrystalline substrate suitable for epitaxial growth of silicon (FIG. 2(iv)).

To prepare the exposed monocrystalline surface of semiconductor substrate 150 for epitaxial growth of silicon, the semiconductor wafer is placed in an evacuated reaction chamber for gaseous reactions. Examples of such reaction chambers include those suitable for low-pressure chemical vapor deposition (LPCVD), reactive ion etching (RIE) and the like, known to those of ordinary skill in the art. The exposed monocrystalline surface of semiconductor substrate 150 may be cleaned, for example, using chlorine, to remove in situ any native oxide on the exposed areas of the semiconductor substrate. This cleaning step may require exposure to a temperature of 800° C. or higher. However, the step may be carried out over a very brief period of time, and thus can be accommodated in the junction thermal budget for forming the memory structure.

Thereafter, without removing the semiconductor wafer from the evacuated reaction chamber, selective epitaxial growth of silicon may be carried out, initiated from the cleaned monocrystalline surface of semiconductor substrate 150. FIG. 2(v) shows an intermediate structure with single-crystal silicon 180 during the selective epitaxial growth. Selective epitaxial growth of silicon may be carried out at a relatively low temperature (e.g., 600° C.). In addition, in situ boron doping may be carried out simultaneously, so that the resulting single-crystal silicon would have a p⁻ dopant concentration (e.g., 1.0-50×10¹⁸ cm⁻³). The selective epitaxial growth of silicon is complete when single-crystal silicon 180 completely fills the deep tranches between the stacks of active strips, as shown in FIG. 2(vi). A channel recess step may then anisotropically remove the single crystal silicon 180 from the deep trenches, leaving single-crystal silicon channel regions 181 in the recessed cavities of the active strips, as shown in FIG. 2(vii).

Other methods for providing single-crystal silicon channel regions for storage transistors in a memory structure are also possible. FIGS. 3(i) to 3(viii) illustrate providing single-crystal silicon channels in a memory structure by selective epitaxial growth of silicon from a monocrystalline substrate or from grain boundaries of polysilicon, according to alternative embodiments of the present invention.

FIG. 3(i) shows stacks 101a and 101b of active strips after the NIN etch cits deep trenches into the deposited active layers, as discussed above with respect to FIG. 2(i). According to an alternative embodiment of the present invention, rather than first recessing source layer 103 and drain layer 104, a selective etch of dielectric layer 102 (e.g., an isotropic silicon oxide etch) is carried out to first recess dielectric layer 102, as illustrated in FIG. 3(ii). Thereafter, a portion of isolation layer 151 is removed to expose a portion of the monocrystalline surface of semiconductor substrate 150 for initiation of epitaxial growth of silicon (FIG. 3(iii)). Having made a monocrystalline surface available, the semiconductor wafer may be removed to an evacuated reaction chamber for gaseous reactions, as discussed above with respect to the cleaning and the selective epitaxial growth of silicon steps above.

Without removing the semiconductor wafer from the evacuated reaction chamber, both the cleaning step and the selective epitaxial growth of silicon step are carried out. The selective epitaxial growth of silicon is complete when single-crystal silicon 180 completely fills the deep tranches between the stacks of active strips, as shown in FIG. 3(iv). A channel recess step may then anisotropically remove the single crystal silicon 180 from the deep trenches, leaving single-crystal silicon channel regions 181, as shown in FIG. 3(v).

Alternatively, ater the isotropic oxide etching step of FIG. 3(ii) is carried out, a selective epitaxial growth of silicon may be carried out. Unlike the selective epitaxial growth of silicon discussed above, which are initiated from an exposed monocrystalline silicon surface, the selective epitaxial growth of silicon is initiated at the grain boundaries of the p⁺ polysilicon. The selective epitaxial growth of silicon is complete when single crystal silicon 180 completely fills the deep tranches between the stacks of active strips, as shown in FIG. 3(vii). A channel recess step may then anisotropically remove the single crystal silicon 180 from the deep trenches, leaving single crystal channel regions 181, 181 in the recessed cavities of the active strips, as shown in FIG. 3(viii).

The single-crystal silicon channel regions in thin-film storage transistors, according to the embodiments of the present invention, provide the following advantages:

• • (a) in situ formation of clean source and drain junctions without deleterious effects from their native oxides;
  • (b) high electron mobility in the channel region;
  • (c) high-quality tunnel oxide in the charge-trapping layer may be formed;
  • (d) reduced out-diffusion from the source and drain layers into the channel regions;
  • (e) monocrystalline silicon channel region with desirable grain boundary characteristics; and
  • (f) reduced leakage current in the source and drain junctions.

The above detailed description is provided to illustrate specific embodiments of the present invention and is not intended to be limiting. Numerous modifications and variations within the scope of the present invention are possible. The present invention is set forth in the accompanying claims.

Claims

1. A memory string formed above a planar surface of a semiconductor substrate, comprising: a common drain region formed out of a first strip of polysilicon of a first conductivity extending lengthwise substantially parallel to the planar surface;a common source region formed out of a second strip of polysilicon of the first conductivity extending lengthwise substantially parallel to the common drain region;a strip of insulator formed between the common drain region and the common source region;a plurality of channel regions formed, respectively, out of first and second strips of single-crystal epitaxial-grown silicon doped to a second conductivity opposite the first conductivity, wherein the first and second strips of epitaxial-grown silicon (a) each are provided between the first and the second strips of polysilicon, and (b) are isolated from each other by the strip of insulator;a plurality of conductors each extending lengthwise along a direction substantially normal to the planar surface; anda plurality of charge-trapping regions each abutting both one of the channel regions and one of the conductors, wherein each charge-trapping region, the conductor abutting the charge-trapping region, the channel region abutting the charge-trapping region, the common source region and the common drain region in combination form a storage transistor of the memory string, with the conductor abutting the charge-trapping region serving as a gate electrode.

2. The memory string of claim 1, wherein the memory string is one of a first group of memory strings provided one on top of another along the direction substantially normal to the planar surface and each isolated from another by an isolation material, with each conductor serving as a gate electrode to a storage transistor of the memory string serves as a common gate electrode to two or more storage transistors in the memory strings within the first group of memory strings.

3. The memory string of claim 2, wherein the first group of memory strings is formed separated from a second group of memory strings by a group of the conductors serving as common gate electrodes to storage transistors of the first group of memory strings, the group of conductors also serve as common gate electrodes to storage transistors of the second group of memory strings.

4. The memory string of claim 3, wherein the group of conductors are provided in a trench formed between the first and the second groups of memory strings, the trench being filled by the group of conductors and the charge-trapping regions.

5. The memory string of claim 1, wherein the first and the second strips of single-crystal epitaxial-grown silicon are formed in cavities formed between the common drain region and the common source region as a resulting of etching the strip of insulator.

6. The memory string of claim 1, wherein the each strip of single-crystal epitaxial-grown silicon abuts the common source region and the common drain region without an intervening layer of native silicon oxide.

7. The memory string of claim 1, wherein each strip of single-crystal epitaxial grown silicon is doped in situ to the second conductivity.

8. The memory string of claim 1, wherein the single-crystal epitaxial-grown silicon is formed using a selective epitaxial silicon growth technique.

9. The memory string of claim 1, wherein the selective epitaxial silicon growth technique initiates growth at sites of polysilicon grain boundaries in the first and the second strips of polysilicon.

10. The memory string of claim 1, further comprising a conductive layer adjacent and in contact with each of the first and the second strips of polysilicon.

11. The memory string of claim 1, wherein the strip of insulator comprises silicon nitride.