Methods and arrangements for improved formation of control and floating gates in non-volatile memory semiconductor devices

[0002] The present invention relates to semiconductor devices and manufacturing processes, and more particularly to methods and apparatus associated with non-volatile memory semiconductor devices.

[0003] A continuing trend in semiconductor technology is to build integrated circuits with more and/or faster semiconductor devices. The drive toward this ultra large-scale integration (ULSI) has resulted in continued shrinking of device and circuit features. As the devices and features shrink, new problems are discovered that require new methods of fabrication and/or new arrangements.

[0004] A flash or block erase Electrically Erasable Programmable Read Only Memory (flash EEPROM) semiconductor memory includes an array of memory cells that can be independently programmed and read. The size of each memory cell, and therefore the memory array, is made small by omitting select transistors that would enable the cells to be erased independently. The array of memory cells is typically aligned along a bit line and a word line and erased together as a block. An example of a memory of this type includes individual metal oxide semiconductor (MOS) memory cells, each of which includes a source, drain, floating gate, and control gate to which various voltages are applied to program the cell with a binary 1 or 0. Each memory cell can be read by addressing it via the appropriate word and bit lines.

[0005] An exemplary memory cell 8 is depicted in FIG. 1a.As shown, memory cell 8 is viewed in a cross-section through the bit line. Memory cell 8 includes a doped substrate 12 having a top surface 11, and within which a source 13aand a drain 13bhave been formed by selectively doping regions of substrate 12. A tunnel oxide 15 separates a floating gate 16 from substrate 12. An interpoly dielectric 24 separates floating gate 16 from a control gate 26. Floating gate 16 and control gate 26 are each electrically conductive and typically formed of polysilicon.

[0006] On top of control gate 26 there is a silicide layer 28, which acts to increase the electrical conductivity of control gate 26. Silicide layer 28 is typically a tungsten silicide (e.g., WSi₂), that is formed on top of control gate 26 prior to patterning, using conventional deposition and annealing processes.

[0007] As known to those skilled in the art, memory cell 8 can be programmed, for example, by applying an appropriate programming voltage to control gate 26. Similarly, memory cell 8 can be erased, for example, by applying an appropriate erasure voltage to source 13a.When programmed, floating gate 16 will have a charge corresponding to either a binary 1 or 0. By way of example, floating gate 16 can be programmed to a binary 1 by applying a programming voltage to control gate 26, which causes an electrical charge to build up on floating gate 16. If floating gate does not contain a threshold level of electrical charge, then floating gate 16 represents a binary 0. During erasure, the charge is removed from floating gate 16 by way of the erasure voltage applied to source 13a.

[0008] FIG. 1bdepicts a cross-section of several adjacent memory cells from the perspective of a cross-section through the word line. In FIG. 1b,the cross-section reveals that individual memory cells are separated by isolating regions of silicon dioxide formed on substrate 12. For example, FIG. 1bshows a portion of a floating gate 16aassociated with a first memory cell, a floating gate 16bassociated with a second memory cell, and a floating gate 16cassociated with a third memory cell. Floating gate 16ais physically separated and electrically isolated from floating gate 16bby a field oxide (FOX) 14a.Floating gate 16bis separated from floating gate 16cby a field oxide 14b.Floating gates 16a,16b,and 16care typically formed by selectively patterning a single conformal layer of polysilicon that was deposited over the exposed portions of substrate 12, tunnel oxide 15, and field oxides 14a-b.Interpoly dielectric layer 24 has been conformally deposited over the exposed portions of floating gates 16a-cand field oxide regions 14a-b.Interpoly dielectric layer 24 isolates floating gates 16a-cfrom the next conformal layer which is typically a polysilicon layer that is patterned (e.g., along the bit line) to form control gate 26. Interpoly dielectric layer 24 typically includes a plurality of films, such as, for example, a bottom film of silicon dioxide, a middle film of silicon nitride, and a top film of silicon dioxide. This type of interpoly dielectric layer is commonly referred to as a oxide-nitride-oxide (ONO) layer.

[0009] The continued shrinking of the memory cells, and in particular the features depicted in the memory cells of FIGS. 1a-b,places a burden on the fabrication process to deposit/form the floating gate 16 and control gate 26 without creating deleterious effects within the memory cell. Of particular concern caused by the shrinking dimensions is the need to provide adequate isolation between each of the floating gates 16a-c,and between each of the floating gates 16a-cand control gate 26, while also providing an adequately arranged floating/control gate configuration.

[0010] These needs and others are met by the present invention, which provides methods and arrangements that increase the process control during the fabrication of semiconductor devices, and in particular, during the formation of the floating/control gate configuration in a non-volatile memory semiconductor device. In accordance with one aspect of the present invention, it has been found that, in certain semiconductor arrangements, the topology created by the space between adjacent floating gates (e.g., 16aand 16b,can be so severe in shape (e.g., deep and narrow) that the silicide layer 28 formed on the overlying control gate 26 often contains significant depressions over the space. These significant depressions can lead to cracks in the silicide layer 28 during subsequent thermal processing of the semiconductor device, which tends to stress the silicide layer 28.

[0011] In accordance with one aspect of the present invention, the amount and shape of the open spaces located between floating gates following patterning is reduced and altered to present a modified topology that is not so severe. The modified topology improves the step coverage of both the control gate material and silicide material, and thereby does not cause significant depressions to form in the subsequently formed silicide layer 28. Consequently, cracking of silicide layer 28 is substantially less likely to occur during the subsequent thermal processes.

[0012] Thus, in accordance with certain embodiments of the present invention, a method, and an associated arrangement, are provided. The method includes the step of forming at least two isolating regions, separated by an isolated region, within a substrate. The method further includes the steps of forming a tunnel oxide on the top surface of the substrate and within the isolated region, forming a floating gate over the substrate, forming a dielectric layer on the floating gate and at least a portion of each of the two isolating regions, and forming at least one spacer on a portion of the dielectric layer, wherein the portion of the dielectric layer is at least partially located directly over and contacting one of the two isolating regions.

[0013] The above stated needs are further met by still other embodiments of the present invention that also include a method and associated arrangements. The method for fabricating a semiconductor device includes the step of forming at least two isolating regions, separated by an isolated region, within a substrate. The method further includes the steps of forming a floating gate over the substrate, forming at least one spacer that contacts the floating gate and one of the two isolating regions, and forming a dielectric layer on the floating gate, the spacer and at least a portion of each of the two isolating regions.

[0014] In accordance with still other embodiments of the present invention, there is provided a method for forming a control gate in a non-volatile memory device. The method includes the steps of forming a dielectric layer on a floating gate arranged within a semiconductor device, forming a first layer of conductive material on the dielectric layer, removing a portion of the first layer of conductive material, forming a second layer of conductive material on a remaining portion of the first layer of conductive material, and selectively patterning the second layer and the remaining portion of the first layer of conductive material to form a control gate. In certain embodiments, the step of removing a portion of the first layer further includes removing substantially all of the first layer to form at least one spacer.

[0015] In accordance with certain embodiments of the present invention, a method is also provided for forming a floating gate in a non-volatile memory device. The method includes the steps of forming a first layer of conductive material on a tunnel oxide portion of a semiconductor device, selectively patterning the first layer of conductive material to form an initial floating gate, forming a second layer of conductive material on the initial floating gate and at least a portion of an isolating region of the semiconductor device, and removing at least a portion of the second layer of conductive material to form at least one spacer that contacts the isolating region and the initial floating gate.

[0016] The foregoing and other features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.

[0017] The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements in which;

[0018] FIGS. 1a-bdepict different cross-sectional views of portions of a typical prior art semiconductor device having at least one memory cell;

[0019] FIGS. 2a-2esequentially depict cross-sectional views of a portion of a semiconductor device during the formation of a plurality of memory cells, which results in at least one of the memory cells having a control gate arrangement with a damaged or cracked silicide layer;

[0020] FIG. 3 depicts a cross-sectional view of a portion of a semiconductor device having a plurality of memory cells having spacers in accordance with an embodiment of the present invention;

[0021] FIG. 4 depicts a cross-sectional view of a portion of a semiconductor device having a plurality of memory cells having an additional polysilicon layer, in accordance with an embodiment of the present invention; and

[0022] FIGS. 5a-bsequentially depict a cross-sectional view of a portion of a semiconductor device during the formation of a plurality of memory cells, having spacers arranged on the floating gates in accordance with an embodiment of the present invention.

[0023] The process steps and structures described below do not form a complete process flow for manufacturing integrated circuits. The present invention can be practiced in conjunction with integrated circuit fabrication techniques currently used in the art, and only so much of the commonly practiced process steps are included as are necessary for an understanding of the present invention. The figures representing cross-sections of portions of an integrated circuit device during fabrication are not drawn to scale, but instead are drawn to illustrate the features of the present invention.

[0024] FIG. 2arepresents a portion 10 of a semiconductor device as viewed from a cross-sectional perspective through the word line (similar to FIG. 1b). Portion 10, in FIG. 2a,depicts the formation of floating gates 16a-cfrom at least one layer of polysilicon. As shown, a conformal polysilicon layer 16 has been formed over substrate 12, tunnel oxide 15, and field oxides 14a-bthat form isolating regions. Polysilicon layer 16 can be deposited, for example, using conventional deposition techniques, such as, chemical vapor deposition (CVD) or plasma enhanced CVD (PECVD) techniques. To pattern polysilicon layer 16 into floating gates 16a-16c(see FIG. 2b) a mask 18 has been formed and patterned on polysilicon layer 16. Mask 18, which is typically a lithographic mask, includes openings 20a-20bthat selectively expose portions of the underlying polysilicon layer 16. The formation of floating gates 16a-16cincludes anisotropic etching, such as reactive ion etching (RIE), or other anisotropic plasma etching processes that remove the exposed portions of polysilicon layer 16 through openings 20a-b,and stop on field oxides 14a-b.

[0025] In FIG. 2b,portion 10 has been etched and mask 18 has been stripped away, for example, using conventional stripping techniques, leaving floating gates 16a-c.The etching process creates spaces 22aand 22b.Spaces 22a-22btend to be relatively narrow in width. For example, in sub-micron flash memories, spaces 22aand 22bcan be between about 0.1 and 0.4 microns. Floating gates 16a-care typically about 900 to 1,100 Angstroms thick. Consequently, the critical dimension of spaces 22a-brepresents a significantly severe topology over which the subsequent layers are formed (e.g., interpoly dielectric layer 24, control gate 26 and silicide layer 28).

[0026] FIG. 2cdepicts the portion 10 of FIG. 2bfollowing deposition of interpoly dielectric layer 24 over floating gates 16a-cand within spaces 22aand 22b(see FIG. 2b). In certain preferred embodiments of the present invention, interpoly dielectric layer 24 is an ONO layer. An ONO layer can be formed, for example, by a three stage process in which a first film of silicon dioxide (e.g., about 50 Angstroms thick) is deposited or grown, followed by deposition of a second film of silicon nitride (e.g., about 80 Angstroms thick), and then a third film of silicon dioxide (e.g., about 40 Angstroms thick) is deposited or grown. The ONO layer provides a thin highly-insulative dielectric layer that separates the floating gate 16 from the control gate 26, as depicted in FIG. 2d.

[0027] In FIG. 2d,the portion 10 of FIG. 2chas been further processed to include a conformal layer of polysilicon that is patterned to form the control gate 26. This polysilicon can be deposited to a thickness of about 1,200 Angstroms using conventional deposition techniques, such as, CVD and PECVD techniques. A silicide layer 28, for example tungsten silicide (e.g., WSi₂), is then formed on top of the polysilicon, as depicted in FIG. 2e.Although not visible from the word line cross-sectional perspective of FIG. 2d,the polysilicon is then selectively patterned using conventional polysilicon etching processes to form control gate 26.

[0028] Silicide layer 28 increases the conductivity of the control gate 26 and is designed to carry a significant portion of the programming current during the programming of the memory cells. It has been found, however, that silicide layer 28 can develop cracks, such as cracks 30a-30bin FIG. 2e,during subsequent thermal processing of portion 10. Cracks 30a-30bcan extend partially through silicide layer 28, or in some cases can extend all the way through silicide layer 28 to control gate 26.

[0029] The cracking of silicide layer 28 appears to be caused by a combination of the severe topology of the spaces 22a-b(see FIG. 2b) and the subsequent thermal processing which typically is required to complete the manufacturing process. For example, subsequent thermal processes can include a high voltage anneal or thermal oxidation process associated with subsequent manufacturing steps. Thus, silicide layer 28, when subjected to the high temperatures of the subsequent thermal processes, tends to crack above the severe topology of the underlying layers. Cracks, such as, for example cracks 30aand 30bin FIG. 2e,tend to increase the resistance in the control gate 26 and degrade the performance of the flash memory and/or damage the semiconductor device.

[0030] It has been found that the cracking of silicide layer 28 is more prominent when the spaces 22a-bbetween the floating gates 16a-care less than approximately 0.4 microns. In certain preferred embodiments of the present invention, spaces 22a-bare each approximately 0.26 microns wide, and the thickness of each of the floating gates 16a-cis approximately 900 to 1,100 Angstroms. The resulting topology has been found to cause a significant amount of cracking of silicide layer 28 during subsequent thermal processing. By way of example, in certain situations the cracking of suicide layer 28 has caused the resistance of a word line to increase by 100 times.

[0031] In accordance with the present invention, several new methods and arrangements have been developed to prevent the cracking of silicide layer 28. In accordance with one aspect of the present invention, the cracking of silicide layer 28 is substantially reduced, if not entirely eliminated, by reducing the severity of the underlying topology, and in particular, by altering the shapes of the overlying layers deposited over and above spaces 22a-b.This allows for increased step coverage when forming control gate 26 and silicide layer 28.

[0032] By way of example, FIG. 3 depicts one embodiment of the present invention in which the portion 10 has been modified during fabrication to include spacers 32 on top of interpoly dielectric layer 24. Spacers 32 are formed by depositing a thin layer (e.g., thickness of about 200 Å to about 500 Å of polysilicon over interpoly dielectric layer 24 and etching back the thin layer of polysilicon using a reactive ion etch or plasma etch to etch back portions of the thin layer of polysilicon. As a result of the etch back process, spacers 32 are formed, which alters the overall shape of the topology over spaces 22aand 22bsuch that subsequent deposition of control gate 26 and silicide layer 28 tend to be smoother in shape above spaces 22a-b.As depicted in FIG. 3, only relatively minor depressions 33aand 33bare formed by silicide layer 28 over spaces 22a-b.During subsequent thermal processing, minor depressions 33aand 33bare not, therefore, severe enough to initiate or lead to cracking of silicide layer 28.

[0033] However, in forming spacers 32 there will be contact between the interpoly dielectric layer 24 and the reactive ion or plasma etchant. In certain arrangements, such as when interpoly dielectric layer 24 is an ONO layer, the reactive ion or plasma etchant may damage or otherwise affect the ONO layer. For example, because the upper silicon dioxide film of the ONO layer is typically very thin (e.g., about 40 Angstroms thick), it can be inadvertently removed or substantially thinned such that the control gate 26, when formed, contacts the silicon nitride film of the ONO layer. This could, for example, result in reliability problems if the control gate 26 is poisoned by the nitride in the ONO layer, and/or if the ONO layer electrically breaks down. Thus, it is important to control the etch back process and to select an etching chemistry that provides a high selectivity between the polysilicon and the top film of silicon dioxide.

[0034] To avoid this problem altogether, in accordance with another embodiment of the present invention, the thin polysilicon layer (described above) is only partially etched away after it has been deposited over the interpolydielectric layer 24. Consequently, there is left a thin polysilicon layer 34 (e.g., thickness of about 100 Å to about 500 Å), that has been partially etched back and which also reduces the severity of the topology such that the subsequently formed control gate 26 and silicide layer 28 have only minor depressions 35aand 35b.As before, the minor depressions 35aand 35bare not severe enough to initiate or lead to cracking of silicide layer 28 during subsequent thermal processing.

[0035] In accordance with yet another embodiment of the present invention, the cracking of silicide layer 28, due to the severe topology and subsequent thermal processing, is solved by altering the shape of floating gates 16a-c,for example, by forming the floating gate 16a-cin a multistage process. An exemplary process is shown in FIGS. 5a-b.

[0036] In FIG. 5a,floating gates 16a-chave been formed by patterning an initial polysilicon layer. Next, an additional thin layer (e.g., 200-500 Angstroms thick) of polysilicon 36 has been conformally deposited over floating gates 16a-cand field oxides 14a-b,for example, using the same deposition process used to form the initial polysilicon layer from which floating gates 16a-cwere patterned. The portion 10 in FIG. 5ais then subjected to a reactive ion etch or plasma etch process that is configured to etch back the additional polysilicon layer 36, thereby leaving behind additional polysilicon material, for example, in the form of spacers 38 that contact the sidewalls of the floating gates 16a-c,as shown in FIG. 5b.

[0037] Spacers 38 reduce the severity of the topology of the floating gates 16a-csuch that subsequent deposition of the interpoly dielectric layer 24, and formation of control gate 26 and silicide layer 28 result in a smoother shape. Thus, for example, silicide layer 28 only includes minor depressions 37aand 37b,which are not severe enough to initiate or lead to cracking of silicide layer 28 during subsequent thermal processing.

[0038] Those skilled in the art will recognize that the additional polysilicon spacers (32 and 38) in the disclosed embodiments can be replaced, in certain embodiments, with other materials. For example, in certain embodiments, a dielectric material can be used instead of polysilicon. Further, the deposition and etching of the additional materials for spacers (32 and 38) and layer 34 can be selective; for example, a mask(s) can be used.

[0039] Although the present invention has been described and illustrated in detail, it is to be clearly understood that the same is by way of illustration and example only and is not be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.