Reduction of pad erosion

Abstract: Improved technique of forming trench capacitors without causing excessive erosion at the edges of the array region resulting from polishing. The erosion is reduced by providing a block mask to protect the array region while partially removing a portion of the hard mask used to etch the trenches in the field region. The partial etch equalizes the height of the hard mask in the array and field region after formation of the deep trenches by a reactive ion etch.

Description: Field of the Invention The invention relates to semiconductor fabrication in general and, more particularly, to the reduction of pad film erosion. Background of the Invention In the fabrication of integrated circuits (ICs) or chips, features are created by etching into the silicon substrate. These features are used to form devices. A deep trench (DT), for examtple, serves as a trench capacitor of a memory cell. Typically, in the process for forming a random access memory (RAM) integrated circuit (IC) or chip, an array of deep trenches are etched in the array region of the substrate. Trench capacitors are then formed from the trenches. Such trench capacitors are used to create an array of memory cells, which are interconnected by wordlines and bitlines to form a memory IC. Figs. 1a-1d show a portion of a process for forming the array of trench capacitors. As shown in Fig. 1a, a conventional pad stack 110 is formed on the surface of a substrata 101. The pad stack includes, for example, sequential layers of a pad oxide 112, a pad nitride 114, and a pad hard mask 116 such as TEOS. A layer of photoresist (not shown) is deposited over the pad stack and patterned to selectively expose areas of the pad stack in the array region where the DTs are to be formed. The substrate is then etched by a reactive ion etch (RIE). The RIE erodes the pad TEOS layer at a greater rate in the array region 135 than the non-array field region 130, creating topography. That is, there exists a large step in height between the field and array regions. Subsequently, at least after formation of a node dielectric, the trenches are filled with, for example, heavily doped polysilicon (poly). Since the poly is conformal, the topography created during RIE is transferred to the poly layer as well. In Fig. 1b, a chemical mechanical polish (CMP) is performed to provide a planar surface. The CMP is selective to the hard mask layer, removing the poly without effectively removing the pad TEOS. As can be seen, the CMP erodes the poly in the array region, causing a depression to form therein. This is referred to as dishing. Due to the large step that exists, poly residuals remain at the edges 160 of the array after CMP. After stripping the pad TEOS layer as shown in Fig. 1c, poly "ears" 165 protrude above the pad nitride layer in the Dts. Because of the poly residuals remaining in the edge during the previous CMP step as a result of the topography, the height of the ears at the array edge are higher than those in the middle or plateau portion 162. For example, height at the edge can be > 250nm and the plateau can rage from 0 to 150nm. A touch up CMP is required to remove the poly ears. However, as shown in Fig. 1d, the touch up CMP causes erosion of the pad nitride layer in the array region. Further, the pad nitride erosion is more pronounced at the edge than at the plateau portion of the array region. Such erosion is detrimental since it causes variations in gate threshold voltage and increases buried strap resistance, adversely affecting yield. As apparent from the foregoing discussion, it is desirable to provide an improved pad stack for use in etching DTs. Summary of the Invention The invention relates to fabrication of integrated circuits. In particular, the invention provides an improved technique for forming deep trenches. In one embodiment, a block mask is used to protect the array region while an etch is performed to remove a portion of the hard mask. The depth of the field etch is equal to about the amount of material that is eroded in the array region by the reactive ion etch that forms the deep trenches. As a result, the hard mask is about the same height in both the array and field regions after the formation of the deep trenches. Description of the Drawings Figs. 1a-1d show a conventional process of forming deep trenches; Fig. 2 shows a trench capacitor DRAM cell; and Figs. 3a-f show a process of forming deep trenches in accordance with one embodiment of the invention. Detailed Description of the Invention The invention relates to the fabrication of ICs. Typically, a plurality of ICs are processed in parallel on a semiconductor substrate such as a silicon wafer. Other types of semiconductor wafers are also useful. The wafer is diced after forming the ICs, separating them into individual chips. The chips are then packaged and integrated into, for example, consumer products. Such consumer products include, for example, personal computers, cellular phones, and other electronic products. For purposes of discussion, the invention is described in the context of forming a portion of a chip. In particular, the invention is described in the context of forming trench capacitors employed in memory cells such as DRAM cells. However, the invention is applicable to the fabrication of ICs in general, including random access memories (RAMs), dynamic RAMs (DRAMs), synchronous DRAMs (SDRAMs), and static RAMs (SRAMs). Other ICs include logic devices such as programmable logic arrays, application specific ICs (ASICs), and merged logic-DRAMs (embedded DRAMs). Referring to Fig. 2, a trench capacitor DRAM cell is shown. Such DRAM cell is described in, for example, Nesbit et al., A 0.6 µm 2 256Mb Trench DRAM Cell With Self-Aligned Buried Strap (BEST) , IEDM 93-627. As shown, the DRAM cell includes a trench capacitor 160 formed in a substrate 101. The trench is filled with, typically, polysilicon (poly) 161 that is heavily doped with n-dopants. The poly serves as one plate of the capacitor, is referred to as a "storage node." A buried plate doped with n-type dopants surrounds the lower portion of the trench. In the upper portion of the trench is a collar 168 for reducing parasitic leakage. A node dielectric 163 separates the two places of the capacitor. A buried well 170 comprising n-type dopants is provided to connect the buried plates of the DRAM cells in the array. Above the buried well is a p-well 173. The p-well serves to reduce vertical leakage. The DRAM cell also comprises a transistor. The transistor includes a gate and diffusion regions 113 and 114 comprising n-type dopants. The diffusion regions are referred to as the source and drain. The designation of source and drain depends on the operation of the transistor. Connection of the transistor to the capacitor is achieved via a diffusion region 125, referred to as the "node diffusion". The gate stack, also referred to as the "wordline," typically comprises poly and nitride layers. Alternatively, layer is a polycide layer which comprises silicide, such as molybdenum (MoSi x ), tantalum (TiSi x ), tungsten (WSi x ), titanium (TiSi x ), or cobalt (CoSi x ), over a layer of poly to reduce wordline resistance. In one embodiment, the polycide layer comprises WSi x over poly. A nitride liner covers the gate stack and substrate. The nitride layer and nitride liner serve as an etch or polish stop layer for subsequent processing. A shallow trench isolation (STI) 180 is provided to isolate the DRAM cell from other cells or devices. As shown, a wordline is formed over the trench and isolated therefrom by the STI. Wordline is referred to as the "passing woraline". Such configuration is referred to as a folded bitline architecture. An interlevel dielectric layer 189 is formed over the wordlines. A conductive layer, representing a bitline, is formed over the interlevel dielectric layer. A bitline contact opening 186 is provided in the interlevel dielectric layer to contact the source 113 to the bitline 190. A plurality of such cells is configured in an array. The array of cells is interconnected by wordlines and bitlines. Access to a cell is achieved by activating the cell's corresponding wordline and bitline. Figs. 3a-e show a process of forming an array of trench capacitors. Referring to Fig. 3a, a substrate 301 is provided in which the trench capacitors are to be formed. The substrate, for example, is a silicon wafer. Other semiconductor substrates, such as gallium arsenide, germanium, silicon on insulator (SOI), or other semiconductor materials, are also useful. The substrate, for example, may be lightly or heavily doped with dopants of a pre-determined conductivity to achieve the desired electrical characteristics. A pad stack layer 310 is formed on the surface of the substrate. The pad stack includes, for example, a pad oxide layer 312 formed on the surface of the substrate. The pad oxide is formed by, for example, thermal oxidation. The pad oxide is sufficiently thick to reduce stress and promote adhesion between a pad etch stop layer with the substrate. Above the pad oxide is provided a pad etch stop layer 314. The pad etch stop layer should have sufficient selectivity to the material used to fill the trenches. In one embodiment, the pad etch stop layer comprises silicon nitride (Si 3 N 4 ) due to its relatively low etch rate to the polysilicon used to fill the trenches. Typically, the etch selectivity between the poly and etch stop layers is about 60:1. For polishing, the selectivity is about 300:1. The nitride layer is formed by, for example, low pressure chemical vapor deposition (LPCVD). Other techniques for depositing the nitride layer are also useful. Typically, the pad nitride layer is about 200nm. Above the pad nitride is formed a hard etch mask layer 316. The hard etch mask comprises a material that is sufficiently dense or hard to withstand the ion bombardments of RIE during the formation of deep trenches. The thickness of the hard mask layer should be sufficient to prevent the deep trench etch from exposing the nitride layer. Further, the etch mask should have a higher wet etch rate than that of the pad etch stop layer. Typically, the selectivity is greater than about 80. In one embodiment, the first hard mask layer comprises undoped silicate glass such as TEOS. Other hard mask materials, such as boron doped silicate glass (BSG) or flowable oxide, are also useful. In accordance with the invention, a mask layer is deposited on the pad stack. The mask layer is patterned to create a block 320 that covers an array region of the substrate in which deep trenches are to be created. The block serves as an etch mask to protect the pad stack in the array region from being etched. In one embodiment, the mask layer comprises a resist material. The resist is patterned by selectively exposing the resist with an exposure source and a mask containing the desired pattern. The resist is then developed to remove either the exposed or unexposed portions depending on whether a positive or negative resist is used, resulting in the formation of the block mask 320. Referring to Fig. 3b, the mask layer 316 in regions unprotected by the block mask is partially removed. Removal of the mask layer can be achieved by an RIE. Other etch techniques are also useful. The unprotected regions represent field regions of the substrate. In one embodiment, the depth of the etch is substantially equal to about the amount of height differential of the mask layer between the array and field regions after the DT etch as a result of the different erosion rates between the two regions. Typically, the depth of the etch is about 8nm. Of course, the actual depth cf the field etch may vary depending on the erosion rate differential of the mask layer between the field and array regions. The block mask is removed after the partial field etch of the mask layer. Referring to Fig. 3c, the pad stack 310 is patterned using conventional lithographic techniques. Such techniques include depositing a photoresist layer over the pad stack and selectively exposing it with an exposure source and mask. Depending on whether a positive or negative resist is used, either the exposed or unexposed portions of the resist layer are removed during development, leaving regions of the pad stack unprotected. The unprotected regions correspond to areas where trenches are to be formed. An etch, such as a reactive ion etch (RIE), is performed to create openings in the pad stack in regions unprotected by the resist. The openings expose the substrate below. A RIE then etches the substrate to create the DTs. The RIE erodes the pad mask in the array region. Since the pad mask in the field region has been partially etched to mask the excess erosion of the mask layer in the array region, the top surfaces of the pad mask in the field and array regions are substantially co-planar after the DT etch. As shown, ears 360 of the mask layer that are relatively small in height protrude above the pad mask at the array edge. The width of these features depends on the actual shape of the field etch blocking mask. Optionally, diffusion regions surrounding the lower portion of the trenches are formed, serving as buried plates. Formation of the buried plates is achieved by various known techniques such as providing a source from which dopants diffuse into the substrate. Node dielectric of the capacitors is then formed in the trenches. Referring to Fig. 3d, poly 370 is deposited to fill the trenches. To ensure completely filling the trenches, the poly overfills the trench, resulting in excess poly that covers the surface of the substrate. Since poly is conformal, the ears 375 are present on the surface of the poly layer as a result of ears 360 on the mask layer below. Since ears 360 are relatively small in height, ears 375 are also relatively small in height. Referring to Fig. 3e, the excess poly is polished by, for example, CMP. The CMP is selective to the mask layer, removing the poly and using the mask layer as a stop layer. Since the field-array topography is avoided due to the partial field etch, the CMP is able to remove the poly without leaving any residuals at the edge of the array. In Fig. 3f, the mask layer is removed by, for example, a wet etch. The wet etch is selective to the nitride and poly. The wet etch results in poly ears 380 protruding above the surface of the nitride-layer. Since the previous CMP process avoids leaving poly residuals, the height of the poly layers are substantially equal throughout the array regions. This advantageously eliminates the need of a touch up CMP process. The poly ears 380 are removed by an anisotropic etch used to recess the poly in the trenches to form the collar. As such, erosion of the pad nitride, as shown in Fig. 1d is avoided, resulting in improved yields. At this point, an etch such as an RIE is performed to recess the poly in the trenches to form a dielectric collar therein. After the formation of the collar, the remaining portion of the DRAM is fabricated using conventional techniques, such as those described in Nesbit et al., A 0.6 'µm 2 256Mb Trench DRAM Cell With Self-Aligned Buried Strap (BEST) , IEDM 93-627. This includes, for example, filling the trenches, forming buried straps, defining isolation regions to form the STIs, depositing the various layers comprising the gate stack and patterning these layers to form the gate conductors that represent wordlines, depositing an interlevel dielectric layer, creating contact openings, and forming bitlines.

Claims: A method of reducing excessive erosion in the fabrication of an integrated circuit comprising: providing a substrate with a pad stack (310) formed thereon, the pad stack comprising a hard mask layer (316) over an etch stop layer (314), the substrate including a first region where an array of trench capacitors are to be formed and a second region outside of the first region; forming a block mask (320) to protect the first region; partially etching the hard mask layer (316) in the second region, the first region being protected by the block mask during the partial etch; removing the block mask (320); patterning the pad stack (310) to expose portions of the substrate in the first region where the array of deep trenches are to be formed; etching the substrate to form the deep trenches, wherein the etching erodes a thickness of the hard mask layer (316) in the first region at a greater rate than a thickness of the hard mask layer (316) in the second region, the etching resulting in the hard mask layer having a substantially planar surface in the first and second regions as a result of the partial etching of the hard mask layer in the second region; filling the deep trenches with a material (370) to form a storage node of the trench capacitors; and polishing to remove the excess material to expose the hard mask layer (316).

Inventors: FLIENTER BERTRAND, PLOESSL ROBERT, FLIENTER, BERTRAND, PLOESSL, ROBERT
Applicants: SIEMENS AG, SIEMENS AKTIENGESELLSCHAFT
Application year: 1998
Main classification: H01L21
IPC codes: H01L21, H01L27