WO2008087498A1

WO2008087498A1 - Dram stacked capacitor and its manufacturing method using cmp

Info

Publication number: WO2008087498A1
Application number: PCT/IB2007/050819
Authority: WO
Inventors: Audrey Berthelot; Christian Caillat; Bruce Boeck; Marc Piazza
Original assignee: Stmicroelectronics Crolles 2 Sas; Nxp B.V.; Freescale Semiconductor, Inc.
Priority date: 2007-01-17
Filing date: 2007-01-17
Publication date: 2008-07-24

Abstract

The invention concerns a method of forming capacitors in an integrated circuit structure, the capacitors having bottom plates formed in rows of trenches in an insulating layer, the method including steps of etching each of the bottom plates to a first level below the top of the trench in which it is formed, etching, in a strip traversing the row of trenches, the insulating layer to a second level, depositing a dielectric layer (425), depositing a conducting layer (426, 428), and polishing the device down to the insulating layer, wherein the first and second levels are chosen such that said dielectric layer in the strip, the top capacitor plate in the strip and the bottom capacitor plate are not exposed by the polishing.

Description

DRAM STACKED CAPACITOR AMD ITS MANUFACTURING METHOD USING CMP

FIELD OF THE INVENTION

The present invention relates to a DRAM structure and method of forming the same, and in particular to forming capacitors in a DRAM structure. BACKGROUND TO THE INVENTION

DRAM (Dynamic Random Access Memory) devices are known in the art, and generally comprise elementary cells, each comprising a transistor and a capacitor. The elementary cells are usually arranged in a matrix. Charges are stored on the capacitor, and transferred to/from the capacitor via the transistor. DRAM structures can be formed based on stack technology, in which the capacitor is formed over the bit line (COB) , or under the bit line (CUB) .

Generally, to form such DRAM devices, transistors, for example MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) are formed over a substrate in a layer of a silicon structure, and the corresponding capacitors are formed in a layer above the transistor layer. The capacitors are generally formed in trenches etched in an insulating material, and usually comprise a bottom plate formed of a metal, a dielectric layer, and a top plate also formed of a metal. While the bottom plates of the capacitors are usually isolated from each other, the dielectric layers and the top plates of the capacitors are deposited over the whole device and remain connected as this helps to avoid charging effects.

Figure 1 is a plan view schematically illustrating part of a known DRAM structure. Such a layout is for example disclosed in US Application 2006/0086960. The edges of the bottom capacitor plates are indicated by rectangles 2. A box 4 in the middle of each rectangle 2 is used to designate a contact, which connects the bottom capacitor plate to a source/drain contact in the transistor level below.

Further contacts, for example bit line contacts, are shown labelled 6. Such contacts pass through the capacitor layer, in between the bottom capacitor plates, and allow contact to be made with source/drain contacts of transistors in the transistor layer. It is normal practise to leave a reasonable separation between these contacts and the top and bottom capacitor plates to avoid shorts. The separation is generally provided by etching a hole through the top capacitor plate and dielectric layer around the point where a contact is to be formed. These etched holes are shown by striped regions 8 in Figure 1. As illustrated, a separation Sa is thus provided between the edge of the etched holes 8 and the contacts 6.

Relatively large margins are also provided between the edges of the etched holes 8 and the bottom capacitor plates 2 to reduce the risk of etching in the region of the bottom capacitor plate in the case of etch misalignment. If etching of the top capacitor plate occurs over the bottom capacitor plate, the dielectric material between the capacitor plates risks being degraded, leading to current leakage. This margin is shown labelled Sb in Figure 1.

The DRAM arrangement of Figure 1 has a number of disadvantages. In particular, the margin Sb surrounding each contact uses up valuable chip area, which is a disadvantage when there is a desire in the industry to form DRAM structures of reduced size and containing an increased number of cells. Additionally, the etching process for etching through the top capacitor plate and the dielectric layer is difficult to perform, for example because small holes and large openings are etched at the same time (using a clear mask) . There is also a manufacturing difficulty in the control of small hole dimensions and providing the overlay requirements between layers, due to tool limitations. SUMMARY OF THE INVENTION

Embodiments of the present invention provide an integrated circuit structure and method of manufacturing the same that at least partially addresses one or more problems in the prior art .

According to one aspect of the present invention, there is provided a method of forming capacitors in an integrated circuit structure, the capacitors comprising bottom plates formed in rows of trenches in an insulating layer, the method comprising steps of etching each of the bottom plates to a first level below the top of the trench in which it is formed, etching, in a strip traversing the row of trenches, the insulating layer to a second level, depositing a dielectric layer, depositing a conducting layer, and polishing the device down to the insulating layer, wherein the first and second levels are chosen such that the dielectric layer in the strip, the top capacitor plate in the strip and the bottom capacitor plate are not exposed by the polishing.

According to a further aspect of the present invention, there is provided a method of forming a DRAM structure comprising forming capacitors according to the above method, the capacitors being formed over a layer comprising transistors.

According to one embodiment of the present invention, after depositing the conducting layer and before performing the polishing, the method comprises depositing a capping layer over the conducting layer. According to one embodiment of the present invention, the first level is higher than the second level.

According to one embodiment of the present invention, the first and second levels are chosen such that the spacing between each of the first and second levels and the polished surface of the insulating layer is greater than the combined depth of the dielectric layer and the conducting layer.

According to one embodiment of the present invention, the method further comprises depositing an oxide layer of a determined thickness over the device.

According to one embodiment of the present invention, the method further comprises forming a contact traversing the second layer in the insulating layer between adjacent bottom plates of the capacitors. According to a further aspect of the present invention, there is provided an integrated circuit structure comprising capacitors comprising bottom plates formed in rows of trenches in an insulating layer, wherein the bottom plates extend to a level lower than the top of the trench in which they are formed, and wherein the surface of the insulating layer has been lowered in a strip traversing a plurality of the trenches, and a dielectric layer and a top plate of the capacitors are formed over the bottom plates and in the strip such that top plates of the capacitors are connected between the trenches by the step.

According to one embodiment of the present invention, the capacitors are part of a DRAM structure formed over a layer comprising transistors. BRIEF DESCRIPTION OF THE DRAWINGS The foregoing and other purposes, features, aspects and advantages of the invention will become apparent from the following detailed description of embodiments, given by way of illustration and not limitation with reference to the accompanying drawings, in which: Figure 1 (described above) is a plan view schematically illustrating a known layout of a DRAM structure;

Figure 2 is a plan view schematically illustrating a layout of capacitors in a DRAM cell according to one embodiment of the present invention;

Figures 3A to 3F are views schematically illustrating, by means of a first cross-section, successive steps in a method for forming the capacitors in a DRAM structure according to an embodiment of the present invention; and Figures 4A to 4F are views schematically illustrating, by means of a second cross-section perpendicular to the first cross section shown in Figures 3A to 3F, the same successive steps of a method according to an embodiment of the present invention. As is normal in the representation of integrated circuits, the various figures are not drawn to scale. DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Figure 2 is a plan view schematically illustrating a DRAM layout in which the top plates of the capacitors are connected in strips. Rectangles 202 represent the outer edges of bottom capacitor plates. Boxes 204 within each rectangle 202 designate the underlying contacts connecting the capacitor plates to transistor terminals in a transistor layer below. Boxes 206 designate contacts, for example bit line contacts that extend through both the capacitor layer and the transistor layer, to make contact with the transistor terminals.

As illustrated in Figure 2, in this layout the bottom capacitor plates 202 are arranged in rows, and the contacts 206 are provided in rows between the capacitor rows, positioned in between alternate bottom capacitor plates in adjacent rows. As illustrated, a minimum spacing Sa is maintained between contacts 206 and adjacent bottom capacitor plates 202.

Diagonal striped regions 208 designate strips, in other words rectangular regions extending in rows, in which the top capacitor plates of the capacitors are connected. Regions 208 are for example between 0,1 and 0,5 μm wide, and preferably 0,3 μm wide. In regions 210 between the strips 208 the top capacitor plate not formed over the bottom capacitor plates has been removed, allowing contacts 206 to be formed in these regions. Advantageously, as will be described in more detail below, the top capacitor plates have been removed by CMP (chemical mechanical polishing) rather than etching. Regions 210 preferably overlap the bottom capacitor plate by a short distance. A method of forming the DRAM structure of Figure 2 will now be described with reference to Figures 3A to 3F, which show cross-section views, the cross-section taken through a strip 208 as indicated by dashed line "X" of Figure 2, and with reference to Figures 4A to 4F, which show cross-section views, the cross-section taken perpendicular to strips 208 and passing through bottom capacitor plates 202 as indicated by dashed line "Y" in Figure 3.

With reference to Figures 3A and 4A, a layer 402 of the DRAM structure has been formed, comprising a number of transistors (shown in cross-section of Figure 4A) . In this example the transistors are MOSFETs (Metal Oxide Semiconductor Field Effect Transistors), although in alternative embodiments other types of FETs or different types of transistors could be provided. A layer 404 has been formed over the transistor layer 402. As shown in Figure 4A, each MOSFET in layer 402 comprises a gate stack 406, spacers 408 formed on either side of the gate stack 406, and a gate contact layer 410, which is for example a silicide layer or salicide (self-aligned silicide) layer, formed over the gate stack 406. The transistors are formed over a substrate (not shown in the figures) and an insulating layer 412 has been formed over the substrate covering the transistors.

A number of contacts 413 are formed traversing the insulating layer 412 and making contact with the source/drain contacts of the MOSFETs on the substrate. Each contact is formed in a hole etched through the insulating layer 412, and comprises a barrier 414 lining the hole, and a contact material 416 filling the hole. The barrier 414 is for example formed of Ti, TiN or WN, while the contact material 416 is for example tungsten. An etch stop layer 417 has then been formed over the device, and a further insulating layer 418, for example of silicon oxide, has been formed over the etch stop layer 417.

Above certain contacts 413 of layer 402 trenches 420 have been etched in insulating layer 418, for example by means of a process involving a photolithography and dry etch step. Each trench 420 is aligned over a corresponding contact 413. A metal layer 422 has then been deposited over the structure, covering insulating layer 418 and lining trenches 420. Metal layer 422 is for example formed of TiN, deposited by ALD (Atomic Layer Deposition) . Trenches 420 are for example rectangular, having widths shown in Figure 3A of a smaller dimension than their lengths shown in Figure 4A.

Figures 3B and 4B illustrate subsequent steps in which a resist layer 424 is deposited filling trenches 420, resist layer 424 protecting portions of the bottom capacitor plate 422 during subsequent etching. Resist 424 can be a non- photosensitive resist. An etch back process is then performed using an etch to selectively etch back the resist and bottom capacitor plate 422 down to a level shown by the dashed line labelled Ll. This level is below the surface of insulating layer

418 and is for example below the top of the trenches 420 by between 10 percent and 30 percent of the total trench 420 depth. Assuming a trench depth of 300 nm, the top of the bottom capacitor plate 422 is for example between 30 nm and 90 nm below the top of the trench.

With reference to Figures 3C and 4C, another etching step is performed, for example comprising the use of photolithography, an optional hard mask, and dry etch step, to etch the insulating layer 418 between the bottom capacitor plates 422 in strips within the width of the trenches 420 of Figure 4B, to reduce the oxide height between trenches 420 in the cross-section as shown in Figure 3C. In particular, this etch is performed in the strips 208 shown in Figure 2. The insulating layer 418 is lowered to a level L2 shown in Figure 3C, which is lower than the height of the bottom capacitor electrodes 422. The insulating layer has been labelled 418' in these regions. In an alternative embodiment, the etching can be performed to lower the insulating layer in these regions to the same height as the bottom capacitor electrodes 422, or to an alternative level below the surface of the insulating layer 418 in the rest of the DRAM.

Next, as shown in Figures 3D and 4D, a dielectric layer 425 is deposited over the device, lining trenches 420 and covering the bottom capacitor electrode 422 in each trench. The dielectric layer 425 also lines the intermediate trenches between adjacent bottom capacitor electrodes above the lowered insulating layer regions 418'. Dielectric layer 425 is for example formed of a high K material deposited by ALD (Atomic Layer Deposition) . Next, a metal layer 426 is deposited covering the dielectric layer 425. Metal layer 426 is for example formed of TiN, and forms the top capacitor plate of the device. The top capacitor plate covers the dielectric layer 425 and due to the narrow width of the trenches in the X direction shown in Figure 3D, which already contain the bottom capacitor plate 422 and the dielectric layer 425, the top capacitor plate 426 fills the remaining space in trenches 420. In alternative embodiments however, trenches 420 are wider, and thus there is still space left in the trenches once the top capacitor plate has been deposited. A capping layer 428 is then deposited, for example formed of tungsten, filling the trenches 420, and forming a layer over the device.

In alternative embodiments, top capacitor plate 426 and the capping layer 428 could be combined in a single layer filling the trenches and covering the device, for example formed of tungsten.

As shown in Figures 3E and 4E, in a next step, CMP is performed to remove portions of the capping layer 428, top capacitor plate 426 and dielectric layer 425 down to the level of the insulating layer 418. The top of the device is thus planarized, for example to the level of the surface of insulating layer 418, or slightly below this. As shown in Figure 4E, in the "Y" cross-section, this involves removing the dielectric layer 425, top capacitor plate 426 and capping layer 428 which have been formed outside of the trenches 420, and thus exposing the top surface of insulating layer 418. However, as shown in Figure 3E, due to the etchback of the lower capacitor plate 422 and of the insulating layers regions 418' between the low capacitor plates in the "X" cross-section, the dielectric layer 425, top capacitor plate 426 and a certain thickness of the capping layer 428 are all lower than the surface of the insulating layer 418 over the rest of the device, the level of which is indicated by dashed line 430 in Figure 3E. Thus these layers are not removed by the CMP, but remain connecting the top capacitor plates in these regions.

Figures 3F and 4F illustrate next steps, in which a contact can be formed through insulating layer 418 to make a connection with contact 413', which is for example a bit line contact or a contact connected to logic portions as labelled 206 in Figure 2. This involves depositing a layer of oxide 432 over the device. An etching step is then performed to etch a hole, aligned with the contact 413', through the oxide layer 432, the insulating layer 418, and the etch stop layer 417, to exposed the top surface of contact 413'. A barrier layer 436 is then deposited over the device, lining the hole 434, and a further layer of metal 438 is deposited over the barrier layer 436, filling hole 434. Excess material from the barrier layer 436 and metal layer 438 deposited outside of hole 434 can be removed, for example using a CMP process, leaving some or all of the oxide layer 432.

Thus, the formation of a DRAM structure has been described in which the bottom capacitor plate in each capacitor has been advantageously lowered allowing CMP to be performed at the level of the top of the trenches in which the bottom capacitor plates have been formed. Thus CMP can be performed without the risk of exposing the bottom capacitor plates, and thus degrading the dielectric layer between the top and bottom plates. The top capacitor plates are advantageously interconnected after the CMP step by lowering the insulating layer between the capacitors in strips, and forming the dielectric layer and top capacitor plates in these strips such that they are lower than the surface of the insulating layer elsewhere in the structure. Furthermore, by etching the insulating layer in these strips to a level L2 lower than the top level of the bottom capacitor plates, the capacitive surface area and thus the capacitance of the capacitors can advantageously be increased by this step. Advantageously, according to the DRAM fabrication method described above, etching holes in the top capacitor plate and dielectric layer, which is performed in prior art techniques, can be avoided.

In some prior methods, after etching the capping layer, top electrode and dielectric layer, an oxide film is deposited and a CMP process is then used to planarize the surface. The control of the remaining oxide thickness over the top electrode and capping layer is susceptible to CMP process variations. As a consequence, thickness limits of the final oxide layer are chosen to avoid shorts between the top electrode and a first metal layer above the oxide layer. This can result in a relatively thick oxide layer, and thus a high aspect ratio of the contact .

The integration method according to embodiments of the present invention described herein advantageously uses a CMP process to form the top electrode, and therefore a standard CMP of the oxide layer is no longer necessary. With a planarized top electrode, the oxide thickness is a direct result of the deposition step, allowing much better control. Advantageously, the aspect ratio of the contact can then be reduced and/or the cylinder height increased (For example the height of contact 413 '/440 of Figure 4F).

The etching step described above in relation to Figures 3C and 4C to lower the level of the insulating layer in regions 208 can be advantageously performed using a dark mask since only strips need to be opened. Defectiveness in logic areas and the control of dimensions are advantageously improved with a dark mask .

Having thus described at least one illustrative embodiment of the invention, various alterations, modifications and improvements will readily occur to those skilled in the art. Such alterations, modifications and improvements are intended to be within the scope of the invention.

For example, whilst an example of a layout of the capacitors and in particular of the etched strips by which top plates of the capacitors are joined has been provided, other arrangements of these strips are possible. For example they could be arranged rotated at 90 degrees to the given example, joining capacitors in the perpendicular direction, or lateral strips could be provided to join the capacitor top plates in both X and Y directions.

The DRAM structure described above is for example part of an embedded DRAM in a device comprising further logic. However, while embodiments have been described applied to a DRAM device, it will be apparent that the methods described herein could be applied to other types of structures having a capacitor layer. For example, the described embodiments could be applied to MEM (micro electro-mechanical) structures, RF MIM capacitors, decoupling capacitors, etc. While the contact material used for the contacts 413 have been described as being tungsten, in alternative embodiments there are many other possible materials, such as copper, metal alloys, metal suicide, etc. Likewise, the top and bottom capacitor plates 422, 426 could be formed of alternative materials to TiN, for example copper, tungsten (W) , tantalum nitride (TaN) , tungsten carbon nitride (WCN) or ruthenium (Ru) etc. The capping layer 428 may be formed of a variety of conducting materials such as tungsten (W), copper etc. The dielectric layer 425 could for example be formed of one or more of silicon dioxide (SiO₂), silicon nitride (SiN₄), tantalum oxide (Ta₂O₅), aluminium oxide (AI2O3) , hafnium oxide (HfO₂), zirconium oxide (ZrO₂) , BST (barium strontium titanate) oxide, or PZT (lead zirconate titanate) .

Claims

CIAIMS

1. A method of forming capacitors in an integrated circuit structure, the capacitors comprising bottom plates (422) formed in rows of trenches (420) in an insulating layer (418), the method comprising: etching each of said bottom plates to a first level below the top of the trench in which it is formed; etching, in a strip traversing said row of trenches, said insulating layer to a second level; depositing a dielectric layer (425) ; depositing a conducting layer (426, 428); and polishing the device down to the insulating layer, wherein said first and second levels are chosen such that said dielectric layer in said strip, said top capacitor plate in said strip and said bottom capacitor plate are not exposed by said polishing.

2. A method of forming a DRAM structure comprising forming capacitors according to the method of claim 1, said capacitors being formed over a layer (202) comprising transistors.

3. The method of any preceding claim, further comprising, after depositing said conducting layer and before performing said polishing, depositing a capping layer (428) over said conducting layer.

4. The method of any preceding claim, wherein said first level is higher than said second level.

5. The method of any preceding claim, wherein said first and second levels are chosen such that the spacing between each of said first and second levels and the polished surface of said insulating layer is greater than the combined depth of said dielectric layer and said conducting layer.

6. The method of any preceding claim, further comprising depositing an oxide layer (432) of a determined thickness over the device.

7. The method of any preceding claim, further comprising forming a contact traversing said second layer in the insulating layer between adjacent bottom plates of the capacitors.

8. An integrated circuit structure comprising capacitors comprising bottom plates (422) formed in rows of trenches (420) in an insulating layer (418), wherein said bottom plates extend to a level lower than the top of the trench in which they are formed, and wherein the surface of the insulating layer has been lowered in a strip traversing a plurality of said trenches, and a dielectric layer (425) and a top plate of said capacitors (426, 428) are formed over said bottom plates and in said strip such that top plates of said capacitors are connected between said trenches by said step.

9. The integrated circuit of claim 8, wherein said capacitors are part of a DRAM structure formed over a layer (402) comprising transistors.