This is a divisional of application Ser. No. 11/092,500, entitled “SEMICONDUCTOR MEMORY DEVICE”, filed on Mar. 29, 2005, which claims priority to U.S. Pat. No. 7,052,941 filed on Jun. 21, 2004, the contents of both of which are incorporated herein by reference.

1. Field of the Invention

This invention relates generally to semiconductor circuitry and, more particularly, to semiconductor memory devices.

2. Description of the Related Art

Advances in semiconductor manufacturing technology have provided computer chips with integrated circuits that include many millions of active and passive electronic devices, along with the interconnects to provide the desired circuit connections. As is well-known, most integrated circuits include laterally oriented active and passive electronic devices that are carried on a single major surface of a substrate. Active devices typically include transistors and passive devices typically include resistors, capacitors, and inductors. However, these laterally oriented devices consume significant amounts of chip area.

It is desirable to provide computer chips that can operate faster so that they can process more data in a given amount of time. The speed of operation of a computer chip is typically measured in the number of instructions per second it can perform. Computer chips can be made to process more data in a given amount of time in several ways. In one way, the number of devices included in the computer chip can be increased so that it can operate faster because more information can be processed in a given period of time. For example, if one computer chip operates on 32-bit data, then another computer chip that operates on 64-bit data can process information twice as fast because it can perform more instructions per second. However, the 64-bit computer chip will need more devices since there are more bits to process at a given time.

The number of devices can be increased by making the devices included therein smaller, but this requires advances in lithography and increasingly expensive manufacturing equipment. The number of devices can also be increased by keeping their size the same, but increasing the area of the computer chip. However, the yield of the computer chips fabricated in a run decreases as their area increases, which increases the overall cost.

Computer chips can also be made faster by decreasing the time it takes to perform certain tasks, such as storing and retrieving information to and from memory. The time needed to store and retrieve information to and from memory can be decreased by embedding the memory with the computer chip on the same surface as the other devices. However, there are several problems with doing this. One problem is that the masks used to fabricate the memory devices are not compatible with the masks used to fabricate the other devices on the computer chip. Hence, it is more complex and expensive to fabricate a computer chip with memory embedded in this way. Another problem is that memory devices tend to be large and occupy a significant amount of area. Hence, if most of the area on the computer chip is occupied by memory devices, then there is less area for the other devices. The total area of the computer chip can be increased, but as discussed above, this decreases the yield and increases the cost.

Accordingly, it is highly desirable to provide new structures and methods for fabricating computer chips which operate faster and are cost effective to fabricate.

The present invention provides a method of forming a circuit which includes providing a substrate; providing an interconnect region positioned on the substrate; bonding a device structure to a surface of the interconnect region; and processing the device structure to form a first stack of layers on the interconnect region and a second stack of layers on the first stack. The width of the first stack is greater than the width of the second stack.

The present invention also provides a semiconductor device which includes a first stack of material layers. A second stack of material layers is positioned on the first stack, wherein the first and second stacks have different widths. First and second control terminals coupled to the first and second stacks, respectively, so that the first and second stacks each operate as an electronic device. One of the first and second stacks operates as a transistor and the other one operates as a negative differential resistance device.

The present invention further provides a circuit which includes a substrate which carries electronic devices. An interconnect region is carried by the substrate, wherein the interconnect region has interconnects coupled to the electronic devices. A device structure is positioned on an upper surface of the interconnect region. The device structure has a first stack of layers positioned on a second stack of layers, wherein the first stack has a width different from the second stack, The device structure is electrically coupled to the electronic devices through the interconnects.

These and other features, aspects, and advantages of the present invention will become better understood with reference to the following drawings, description, and claims.

FIGS. 1-11 are simplified sectional views of steps in the fabrication of a memory device in accordance with the present invention.

FIG. 12 is simplified sectional views of a memory device having tapered slope in its body in accordance with the present invention.

FIGS. 13-14 are simplified sectional views of steps in the fabrication of a memory device with narrowed semiconductor width in accordance with the present invention.

FIGS. 1-12 are simplified sectional views of steps in fabricating a semiconductor memory circuit 100 in accordance with the present invention. In the following figures, like reference characters indicate corresponding elements throughout the several views. In FIGS. 1-12, only a few memory devices are shown in circuit 100, but it should be understood that circuit 100 generally includes a number of memory devices and that only a few are shown for simplicity and ease of discussion.

Circuit 100 can be included in a computer chip where the memory devices are positioned above the computer circuitry. The memory devices are typically coupled to the computer circuitry through interconnects which include a conductive line and/or a conductive via. Circuit 100 has several advantages. One advantage is that the memory devices are positioned above the computer circuitry which is desirable since the memory devices typically occupy much more area than the computer circuitry. Another advantage of circuit 100 is that the memory devices are positioned closer to the computer circuitry so that signals can flow therebetween in less time. Still another advantage of circuit 100 is that the computer circuitry are fabricated with a different mask set than the memory devices. Hence, the masks are less complicated and less expensive to make. A further advantage is that the memory devices are fabricated from blanket semiconductor layers after they have been bonded to the interconnect region. Hence, the memory devices do not need to be aligned with the computer circuitry, which is a complicated and expensive process.

In FIG. 1, partially fabricated circuit 100 includes an interconnect region 131 carried by a substrate 130. Interconnect region 131 provides support for structure positioned thereon its surface 131 a. Interconnect region 131 includes a dielectric material region 133 with interconnect lines 132 and conductive vias 134. Dielectric material region 133 can be formed using many different methods, such as CVD (Chemical Vapor Deposition) and SOG (Spin On Glass). Typically interconnect lines 132 and vias 134 are coupled to electronic circuitry (not shown) carried by substrate 130 near a surface 130 a. Interconnect lines 132 and vias 134 include conductive materials, such as aluminum, copper, tungsten, tungsten silicide, titanium, titanium silicide, tantalum, and doped polysilicon, among others.

A conductive contact region 121 is positioned on surface 131 a of region 131. Region 121 can include one or more material layers, however, it is shown here as including one layer for simplicity. A device structure 101 is positioned on surface 121 a of conductive region 121. In accordance with the invention, structure 101 is bonded thereto surface 121 a using wafer bonding. More information on wafer bonding can be found in co-pending U.S. patent application titled “WAFER BONDING METHOD” and “SEMICONDUCTOR BONDING AND LAYER TRANSFER METHOD” filed on the same date herewith by the same inventor and incorporated herein by reference.

In this embodiment, device structure 101 includes a stack of semiconductor layers which include an n⁺-type doped layer 124 a with a p-type doped layer 124 b positioned on it. An n⁺-type doped layer 124 c is positioned on layer 124 b and a p-type doped layer 124 d is positioned on layer 124 c. An n-type doped layer 124 e is positioned on layer 124 d and a p⁺-type doped layer 124 f is positioned on layer 124 e. In this embodiment, these layers can be doped using diffusion doping, epitaxial growth, ion implantation, plasma doping, or combinations thereof. More information on wafer bonding can be found in a co-pending U.S. patent application titled “SEMICONDUCTOR LAYER STRUCTURE AND METHOD OF MAKING THE SAME” filed on the same date herewith by the same inventor and incorporated herein by reference. In this invention, device structure 101 preferably includes single crystalline material which can have localized defects, but is generally of better quality than amorphous or polycrystalline material.

It should be noted that device structure 101 will be processed further, as shown in FIGS. 2-12, to form one or more desired device(s) which can be many different types. For example, the device(s) can include a memory device, such as a capacitorless Dynamic Random Access Memory (DRAM) device. In this particular example, the electronic device(s) include a Negative Differential Resistance (NDR) type Static Random Access Memory (SRAM) device, which has vertically and serially connected a thyristor and a MOSFET (Metal-Oxide Semiconductor Field-Effect-Transistor). As will be discussed in more detail below, the NDR SRAM device can operate faster and is more stable than a planar NDR SRAM device.

In FIG. 2, a hardmask region 125 is positioned on a surface 101 a of device structure 101 and a photoresist region 126 is positioned on hardmask region 125. Hardmask region 125 can include dielectric materials, such as silicon oxide and silicon nitride. Hardmask region 125 can also include anti-reflective films, such as high-K SiON, in order to reduce reflection during photo process. Photoresist region 126 is patterned and exposed using a photo mask (not shown) so that portions of it can be removed and other portions (shown) remain on hardmask region 125. Photoresist region 126 defines a top portion of the device to be fabricated, as indicated by a dotted line 101 d. In FIG. 3, device structure 101 is partially etched in a known manner to form stacks 127 and photoresist region 126 is removed. Stacks 127 are formed because the etch does not substantially remove the material in region 101 below hardmask region 125.

In FIG. 4, mask regions 128 are positioned around each stack 127. Mask regions 128 extend from a surface 129 a of layer 124 c to mask region 125 of each stack 127. Sidewall mask region 128 can include a dielectric material, such as oxide and/or nitride, deposited by CVD (Chemical Vapor Deposition), and dry etched to form the sidewall. Sidewall mask region 128 and mask 125 protect stack 127 and a portion of a surface 129 a from a subsequent etch step, as will be discussed presently.

In FIG. 5, device structure 101 is etched again to surface 131 a of interconnect region 131 except for portions protected by mask regions 125 and 128. Stack 127 now includes a stack region 127 a positioned on electrode 121, which is electrically connected to interconnect 132 through vias 134. Stack 127 also includes a stack region 127 b positioned on stack 127 a. In this example, stack 127 a is wider than stack 127 b so that a ledge 129 is formed therein stack 127. Here, stack 127 a has a width W₁ and stack 127 b has a width W₂ where W₁ is greater than W₂.

Stack regions 127 a and 127 b include layers of semiconductor materials stacked on top of each other and are defined by sidewalls 119 a and 119 b, respectively. Hence, the devices formed from stacks 127 a and 127 b are called “vertical” devices because their layer structure and sidewalls 119 a and 119 b extend substantially perpendicular to surface 131 a. In other words, the layers of stack 127 are on top of each other so that current flow through pn junctions included therein is substantially perpendicular to surface 131 a and parallel to sidewalls 119 a and 119 b.

This is different from conventional devices which are often called lateral or planar devices. Lateral devices have their layer structure extending horizontally relative to a surface of a material region that carries them. In other words, the pn junctions included in a lateral device are positioned side-by-side so that current flow through them is substantially parallel to the supporting surface.

In FIG. 6, a dielectric material region 133 a is deposited on interconnect region 131, planarized, and etched back so that it partially surrounds stacks 127. In this embodiment, material region 133 a extends up stacks 127 to layers 124 a. Material region 133 a is processed so that it covers electrodes 121 and prevents oxidation during a gate oxidation process, as will be discussed presently.

A dielectric region 123 is deposited around an outer periphery of each stack 127. Dielectric region 123 can include silicon dioxide, silicon nitride, or combinations thereof. It can also include high dielectric constant (high-k) materials, such as Al₂O₃, ZrO₂, HfO₂, Y₂O₃, La₂O₃, Ta₂O₅, TiO₂, and BST (Barium Strontium Titanate). Region 123 can be thermally grown or deposited using thermally evaporation, chemical vapor deposition, physical vapor deposition, or atomic layer deposition. It is beneficial if the thermal growth or deposition can be done using a temperature below about 500° C. so that electrode 121, interconnect region 131, and the electronic circuitry carried by substrate 130 are not damaged or undesirably changed.

In FIG. 7, a conductive region 140 is positioned on stacks 127 so that it surrounds them. Region 140 is positioned on dielectric material region 133 a, hardmask region 125, and dielectric region 123 of each stack. Conductive region 140 can include the same or similar material as those included in vias 134, interconnects 132, and/or region 121. Conductive region 140 operates as a control terminal to modulate the current flow through stack 127. Also, dielectric layer 133 a separates bottom electrodes 121 from each adjacent stack 127. Conductive region 140 c between stacks 127 is thicker than conductive region 140 d because during the deposition process, more conductive material is deposited between adjacent stacks 127.

In FIG. 8, conductive region 140 is partially etched away so that portions surrounding stacks 127 remains. After etching, a portion 140 a of conductive region 140 remains on region 133 a and extends up stack 127 a and a portion of conductive region 140 b remains on surface 129 a of ledge 129 and extends up stack 127 b. The etching can be done by anisotropic etching, such as dry etching, and is done in such a way that conductive regions 140 a and 140 b are not coupled together. Portion 140 a couples each adjacent stack 127 a together. However, portions 140 b of each adjacent stack are not coupled together.

In FIG. 9, a dielectric material region 133 b is deposited on dielectric material region 133 a and can include the same material. Material region 133 b extends up stack 127 a to stack 127 b. A conductive region 141 is positioned on dielectric material region 133 b so that it surrounds stacks 127 b. In FIG. 10, conductive region 141 is partially etched away so that portions 141 a remain around conductive region 140 b and dielectric region 123.

In FIG. 11, a dielectric region 133 c is deposited on dielectric region 133 b so that it surrounds stacks 127. Dielectric region 133 c can include the same material as regions 133, 133 a, and/or 133 b. Trenches are formed through portions of dielectric region 133 c to p⁺-type region 124 f of each stack 127. Contacts 142 are then formed therein so that they extend to a surface 101 a of structure 101. A conductive interconnect 143 is formed on surface 101 a and is coupled to each via 142. It should be noted that another device structure, similar to device structure 101, can be bonded to conductive interconnect 143 and surface 101 a and processed as described above so that multiple layers of devices structures are carried by interconnect region 131.

It should also be noted that sidewalls 119 a and 119 b of stacks 127 a and 127 b, respectively, are substantially perpendicular to surface 131 a. However, in some embodiments, sidewalls 119 a and/or 119 b can be oriented at an angle, other than 90°, relative to surface 131 a. For example, the angle can be 70° so that the sidewalls of stacks 127 a and 127 b are sloped relative to surface 131 a.

FIG. 12 shows device 100 with stacks 127 a and 127 b wherein sidewall 119 a is sloped and sidewall 119 b is perpendicular to surface 131 a. If the sidewalls are sloped, then it is easier to deposit material between adjacent stacks 127. Further, conductive contact 121 can be made wider so the alignment is easier during device processing. Sloped sidewalls also increase the stability of stacks 127 even though its aspect ratio is high. The aspect ratio is the ratio of the height of stack 127 between conductive contact 121 and layer 124 f relative to its width, W₁.

FIGS. 13 and 14 are simplified sectional views of steps in fabricating a semiconductor memory circuit 102 in accordance with the present invention. In FIG. 13, circuit 102 is the same or similar to device 100 shown in FIG. 2, only hardmask region 125 is thicker. In this embodiment, hardmask region 125 is exposed and overetched so that the portions of hardmask 125 between photoresist region 126 and layer 124 f have widths W₄, which is less than width W₂ as shown in FIG. 5. Overetching undercuts hardmask region 125 so that its width W₄ is less than width W₃. Since width W₄ is made smaller, the width of stack 127 b will also be made smaller, as shown in FIG. 14. Here, stack 127 b is shown in phantom with dotted lines. At this point, region 101 can be etched, as shown in FIG. 4, and the processing can move to the subsequent steps described above.

The present invention is described above with reference to preferred embodiments. However, those skilled in the art will recognize that changes and modifications may be made in the described embodiments without departing from the nature and scope of the present invention. Various further changes and modifications will readily occur to those skilled in the art. To the extent that such modifications and variations do not depart from the spirit of the invention, they are intended to be included within the scope thereof.