Self-aligned PECVD etch mask

[0002] The present invention relates to processing methods in semiconductor device fabrication, and more particularly to methods for forming openings in a substrate, such as contact openings.

[0003] At several stages during wafer fabrication, it is necessary to form contact openings through insulative material to establish electrical communication with the integrated circuitry. To form contact openings, a masking layer such as a photoresist is typically formed over an insulative layer such as silicon oxide layer, and the contact opening is etched through the insulative layer by exposure to a plasma formed in a plasma reactor. A fluorocarbon plasma is typically used to etch silicon dioxide.

[0004] When dry etching a contact opening, the etchants also attack the resist layer causing the resist to gradually erode away, often before the desired depth of the opening is achieved. The erosion of the resist overlying the corners at the mouth of the opening can cause faceting or chamfering of the insulative layer from the contact with the etch gas, resulting in surface roughness and striations in the etch features, and the loss of critical dimensions of the opening being etched. In an array such as a memory cell, contacts are positioned in close proximity to each other, and the erosion and localized breakdown of the photoresist in an etch process can result in the development of notches and other blemishes in the surface of the contact, which can extend to and short circuit an adjacent contact or feature.

[0005] The stringent requirements of the etch process (such as rate, profile control, and selectivity to other films in the structure) may not permit that resist selectivity be made high enough to meet the conflicting manufacturing requirements that a) the resist be thin enough to be easily patterned with the lithography equipment in use, and b) the resist be thick enough to provide adequate masking for the etch process.

[0006] Therefore, a need exists for a method of etching silicon oxide layers to provide high aspect ratio openings that overcomes these problems.

[0007] The present invention provides processing methods for forming via or other contact openings using a self-aligned etch mask of a polymeric material.

[0008] The invention utilizes a self-aligning method to deposit a masking layer for a continued etch of a formed contact opening downwardly in a substrate, thus eliminating the need to something in production to compensate for a process with relatively high resist erosion. Typically, a higher selectivity process can be used. Such a process will be more expensive to operate because it will have a smaller process window and be more sensitive to variations in incoming material. Thus, it will generate more scrap and be more costly to operate. Alternatively, a hard mask under the resist layer may be used. Such a mask might be polysilicon or silicon nitride. These masks are costly to deposit, must be separately etched and subsequently removed.

[0009] The present method applies a polymerizing gas surface-wide over a substrate having a formed opening, and utilizes the height difference, or aspect ratio, between the base of the opening and the surface of the substrate to selectively deposit a polymer mask layer onto the surface of the substrate and the sidewalls of the opening to a limited depth. Thus, the polymer layer self-aligns relative to the pre-etched contact opening and serves as an etch mask, for example, to extend the etch depth to a level that the original photomask thickness cannot support. A feature can be etched by repetitively depositing the mask and etching the feature until the desired depth is achieved. The self-aligned mask can be used to clean the base (i.e., bottom of the partially etched feature), or extend the etch depth of an etched opening while protecting the exposed surface of the substrate and upper corner of the opening.

[0010] In one embodiment of the method of the invention, a self-aligning etch mask is used to continue the etch of an etched feature that is initially formed to a first selected depth in the substrate using a photoresist or other masking layer deposited over a substrate. The first depth of the etched feature is sufficient such that substantially no polymer material is deposited on the substrate at the bottom portion of the etched feature in a subsequent polymer deposition step. Preferably, the aspect ratio of the etched feature (i.e., ratio of the depth to the diameter of the feature) is at least about 0.5, preferably greater than about 0.5, more preferably greater than about 1.

[0011] According to the invention, the self-aligned, etch resistant mask layer is formed over the masking layer and the etch is continued. It is preferred that the self-aligned mask layer is formed over a portion of the sidewalls of the etched feature contiguous with the surface of the substrate to a limited depth into the etched feature, to cover and protect the comers and the sidewalls of the substrate from being eroded during a subsequent etch step.

[0012] Preferably, the self-aligned mask layer is deposited from a polymerizing gas by plasma enhanced chemical vapor deposition using a dual source high density plasma etcher, to form an etch resistant polymer material layer of a predetermined thickness. Exemplary polymerizing gases include fluorocarbons, hydrofluorocarbons, and chlorofluorocarbons, among others. A second etching can then be performed to extend the etched feature downwardly to a selected second depth.

[0013] During the deposition step, the thickness of the self-aligned mask can be controlled and modified to provide an effective covering to prevent etching of the substrate, while maintaining the mouth of the etched feature at a suitable width for a subsequent etch of the feature, by varying the bias power, the source power, and/or the deposition time configurations of the etcher. In addition, the etch selectivity/resistivity of the mask layer can be controlled by varying the setting for the source power of the etcher. The use of power to accomplish the self aligned deposition is preferred because power can be changed almost instantly and is, in most processing tools, well controlled and repeatable. It may be necessary to change other variables, most notably gas flow rate or gas pressure, to accomplish the self aligned deposition.

[0014] The self-aligned mask can thus be deposited on a layer of photoresist or other masking layer to continue the etch of a formed feature in a substrate (e.g., oxide, among others). In another embodiment, the self-aligned mask can also be formed over a masking layer that has been patterned with openings to expose an underlying substrate (e.g., oxide layer) that is to be initially etched to form a feature. Preferably, the openings of the mask layer have an aspect ratio at least about 0.5, preferably greater than about 1, such that polymer material deposited from a polymerizing gas is not deposited onto the substrate at the bottom of the mask opening when the self-aligned mask layer is formed.

[0015] In another embodiment of the method of the invention, a self-aligning etch mask is used to remove an etchable material from the bottom portion of an etched feature, while protecting the exposed surface of the substrate and upper corners of the feature. For example, a substrate having an etched feature and an overlying layer of an etchable material (e.g., silicon nitride, polysilicon) formed over the surface of the substrate including the sidewalls and bottom portion of the feature, can be processed to remove a portion of the etchable layer from the bottom of the etched feature. Preferably, the aspect ratio of the feature is at least about 0.5, preferably greater than about 0.5, more preferably greater than about 1, such that substantially no polymer material is deposited onto the substrate at the bottom portion of the feature in a subsequent polymer deposition step.

[0016] A self-aligned, etch resistant mask layer can then formed to a selected thickness over the upper surface of the etchable layer and, preferably contiguously over the comers of the etchable layer and extending into the etched feature for a limited depth. The mask layer effectively protects the etchable layer formed on the upper surface and along the sidewalls of the etched feature from being removed/damaged in a subsequent etch step. Preferably, substantially no polymeric material is deposited onto the etchable layer overlying the bottom portion of the opening. The self-aligned mask layer can be deposited from a polymerizing gas such as a fluorine-, chlorine-, or hydrocarbon-containing gas, and mixtures thereof, by plasma enhanced chemical vapor deposition using a dual source high density plasma etcher. After applying the self-aligned mask layer, the etchable layer overlying the bottom portion of the etched feature is then removed to expose the substrate on the bottom portion of the etched feature. The feature can also be extended past the etchable layer to result in a feature with sidewalls of differing materials. The thickness of the self-aligned mask can be modified by varying the bias power, the source power, and/or the deposition time configurations of the etcher. The etch selectivity/resistivity of the self-aligned mask layer can be modified by varying the configuration for the source power of the etcher.

[0017] Etching a deep feature utilizing the present invention, requires depositing only a thin photolithographic mask to start the etch and to set the stage for the sequential self-aligned mask deposition and etch sequences to increase the etch depth of the feature. The use of the self-aligned mask greatly simplifies the lithographic process by reducing the required thickness of the photoresist mask.

[0018] Another process capability enabled by the invention is the ability to clean the base or bottom portion of an etched feature, for example, a contact that has damaged silicon at the bottom of the opening, while protecting the exposed surface of the substrate and upper corner of the etched feature. This allows the cleaning step to be anisotropic and conserves the cd budget. It also allows the use of a cleaning agent that could otherwise remove surface material from the exposed surface of the structure.

[0019] Preferred embodiments of the invention are described below with reference to the following accompanying drawings, which are for illustrative purposes only. Throughout the following views, the reference numerals will be used in the drawings, and the same reference numerals will be used throughout the several views and in the description to indicate same or like parts.

[0020] FIG. 1A is a diagrammatic cross-sectional view of a semiconductor wafer fragment at a preliminary step of a processing sequence according to a first embodiment of the method of the present invention.

[0021] FIGS. 1B-1D are views of the wafer fragment of FIG. 1A at subsequent and sequential processing steps.

[0022] FIG. 2A is a diagrammatic cross-sectional view of a semiconductor wafer fragment at a preliminary step of a processing sequence according to another embodiment of the method of the present invention.

[0023] FIGS. 2B-2C) are views of the wafer fragment of FIG. 2A at subsequent and sequential processing steps.

[0024] FIG. 3A is a diagrammatic cross-sectional view of a semiconductor wafer fragment at a preliminary step of a processing sequence according to another embodiment of the method of the present invention.

[0025] FIGS. 3B-3D are views of the wafer fragment of FIG. 3A at subsequent and sequential processing steps.

[0026] FIGS. 4A-4D are SEM photographs of a cross-sectional view of BPSG substrates with openings etched according to the first embodiment of the method of the invention.

[0027] The present invention provides a method for etching of a layer on a semiconductor wafer or other substrate, to provide an etched feature such as a via or contact opening having a high aspect ratio.

[0028] FIGS. 1A-1F depict a first embodiment of the method of the invention, in which an etched feature is formed by first etching an exposed area of an etchable layer (e.g., silicon oxide) through an opening in a photoresist or other patterned mask layer, to a first depth, and then second etching the formed feature to a second depth using a self-aligned polymer mask.

[0029] Referring to FIG. 1A, the wafer fragment 10 includes a substrate 12, and a layer of material 14 formed over the substrate 12 that comprises an etchable material into which an etched feature (e.g., contact opening) is to be formed. An exemplary etchable material for layer 14 is a dielectric or insulating material, for example, silicon dioxide, or a doped silicon dioxide such as phosphosilicate glass (PSG) and borophosphosilicate glass (BPSG), among others, in a single layer or multiple layers. Such dielectric layers typically overlay a silicon-comprising substrate 12 such as single crystal silicon, epitaxial silicon, polysilicon, or silicides such as titanium silicide.

[0030] The etched feature can be initially formed by a conventional etch process that includes forming a masking layer 16, such as a layer of photoresist, over the etchable layer 14 and patterning the masking layer to define an opening 18 that exposes an area 20 of the etchable layer 14 where the feature is to be etched. The masking layer 16 can be deposited, patterned and developed to provide a desired contact pattern using conventional photolithographic techniques as known in the art.

[0031] Referring to FIG. 1B, a first etching of the etchable layer 14 through the mask opening 18 is performed to etch anisotropically a feature 22, (e.g., opening) having substantially vertical sidewalls 24 and a bottom portion 26. The feature 22 is initially etched to a first depth D₁, utilizing the photoresist or other masking layer 16.

[0032] The feature 22 provided by the first etching using the masking layer 16 has a sufficient first depth D₁such that, when the wafer 10 is subsequently exposed to a polymerizing gas, substantially no polymeric material is deposited on the bottom portion 26 of the feature 22. To that end, it is preferred that the aspect ratio (i.e., the height-to-width ratio) of the contact opening 22 is greater than about 0.5, more preferably greater than about 1. For example, a feature having a width of about 0.25 μm, preferably has an etch depth of about 0.125 to about 0.25 μm or greater.

[0033] The etching process and gases used to form the etched feature 22 in the etchable layer 14 are dependant upon the material to be etched and can be performed by conventional etch processes as known and used in the art. Etching of the silicon dioxide layer is preferably by dry plasma etch, which is generally conducted within an etch chamber of a plasma generating device. The plasma etch technique can include the use of reactive ion etching (RIE), plasma etching (PE), or other type of etching technique, for example, electron cyclotron resonance (ECR), magnetically enhanced cyclotron resonance (MERIE), among others, as known and used in the art. Various etch parameters such as the gas mixture, temperature, RF power, pressure, and gas flow rate, among others, can be varied to achieve the desired etch rate and etch characteristics of the plasma system, as also known in the art.

[0034] Useful etch gases are halogen-containing gases including fluorine-containing gases and/or chlorine-containing gases. Fluorine-containing gases are typically used to etch silicon oxides, and include, for example, CF₄, CHF₃, C₂F₆, C₃F₈, C₄F₈, among others. Plasma etching of silicon nitride can be obtained with the use of CF₄/O₂, CHF₃/O₂, CH₂F₂and CH₃F plasmas, among others. A polysilicon or metal layer can be etched using a gas plasma comprising a chlorine- or bromine-based plasma such as HBr, HBr/O₂and HBr/Cl₂/O₂, among others. The polymerizing gas can also include inert carrier gases.

[0035] Although not shown, after etching of the feature 22 to a first depth DI is completed, the masking layer 16 can optionally be stripped to remove a portion or all of the masking layer 16. The photoresist or other masking layer 16 can be removed by a conventional process dependant upon the material used to form the masking layer. Conventional dry etch process to remove a resist is a dry plasma ashing technique by which an oxygen plasma is generated in a plasma etch chamber (i.e., RF plasma machine) to oxidize the resist.

[0036] According to the method of the present invention, a self-aligned mask can then be applied to continue the etch of the formed feature 22 downwardly into the layer of etchable material 14. The self-aligned mask layer 30 can be conformally deposited surface-wide over the wafer (target) surface from a polymerizing gas over the masking layer 16, to result in the structure shown in FIG. 1C. The present method utilizes the height difference, or aspect ratio, between the bottom portion of the feature and the surface of the substrate, i.e., the depth D₁of the feature 22, provided by the first etching to selectively deposit polymer material onto the surface 28 of the masking layer 16 and the sidewalls 24 of the feature 22 to a limited depth. As such, the polymer layer 30 self-aligns relative to the feature 30 and serves as a mask for the subsequent etching of the feature. The wafer 10 is exposed to the polymerizing gas under conditions and a time period effective to deposit the self-aligned mask layer 30 to a predetermined thickness T₁over the exposed surfaces of the wafer including the surface 28 of the masking layer 16.

[0037] As depicted in FIG. 1C, it is preferred that the self-aligned mask layer 30 is formed over the surface 28 of the masking layer 16 and extends into the feature 22 over a portion of the sidewalls 24, preferably in a continuous layer to cover the corners 32 of the etchable layer 14. It is further preferred that the deposition step results in a polymer layer 30 whereby a bulge or lateral growth is formed in the area 34 at the interface between the sidewalls 24 and the surface 28 of the masking layer 16, to cover the comers 32 of the etchable layer 14. This phenomenon is known as “cusping” or “breadloafing.”

[0038] Coverage of the comers 32 of the etchable layer 14 with the self-aligned mask layer 30, particularly the breadloaf area 34, helps protect the sidewalls 24 from lateral etching and prevents etch-off and/or faceting of the corners 32 in a subsequent etching step, which wears away the etchable layer resulting in surface roughness and striations in the etch features, and the loss of critical dimensions of the opening 22 being etched. In an array such as a memory cell, contacts are positioned in close proximity to each other, and the erosion of the etchable 14 layer can result in the development of notches and other blemishes in the surface of the contact or feature, which can extend to and short circuit an adjacent contact.

[0039] Depending on the application, any thickness of the self-aligned mask layer 30 can be useful, but a self-aligned mask layer 30 of about 50 angstroms or greater is preferred for most applications, preferably about 2500 angstroms. The thickness of the mask layer at the breadloaf area 34 should effectively cover the corners of the etchable layer and protect them from being etched, but not project into the opening 22 to substantially constrict the mouth 36 of the feature 22, whereby a subsequent etch of the feature 22 is inhibited, resulting in the profile of the sidewalls becoming tapered.

[0040] It is highly preferred that substantially no deposit (i.e., layer) is formed from the feed gas(es) on the bottom portion 26 of the partially etched of the feature 22 (or other feature) that would inhibit downward etching of the feature 22. This can be controlled, at least in part, by initially etching the feature 22 to an aspect ratio of greater than about 0.5, preferably greater than about 1.0, and/or varying the parameters used in applying the polymerizing gas. For example, to limit the depth of the deposition of the polymerizing gas into the feature 22 using a dual source high density plasma etcher, the following parameters can be controlled: source power, bias power, gas flow rate(s), pressure, and chamber walls and wafer temperatures.

[0041] Polymerizing gases for forming the self-aligned mask layer 30 on a silicon oxide or other etchable layer 14 include fluorocarbons and hydrofluorocarbons. Exemplary gases include CF₄, CHF₃, CH₂F₂, CH₃F, C₂F₆, C₂HF₅, C₃F₆, C₄F₈, and mixtures thereof. Higher molecular weight polymerizing gases as well as hydrocarbons and other halocarbons, for example, CCl₄, can be used. In general, polymerizing gases that are the same as or similar to the etch gases are preferred. Therefore, a chlorocarbon, such as CCl₄, would be preferred in an aluminum etching and a fluorocarbon would be preferred in an oxide etching. Mixed halogen-containing gases such as CFCl₃and CF₂Cl₂, and halogenated hydrocarbons such as C₂HCl₃, among others, and mixtures thereof, are also useful. Examples of other suitable polymerizing gases include iodine-containing gases and higher molecular weight hydrocarbons such as C₂H₄, C₃H₈, C₃H₆, C₄H₆, C₄H₈, and cyclic or aromatic compounds.

[0042] To apply the self-aligned mask layer 30, the wafer 10 of FIG. 1B can be placed into a chamber of a plasma source such as a conventional plasma etcher or a high density plasma etcher (not shown), and exposed to a polymerizing gas using a conventional plasma enhanced vapor deposition (PECVD) process. High density plasma etchers operate at pressures below 50 mtorr (typically below 25 mtorr) and have plasma densities greater than 10¹⁰to 10¹¹cm⁻³. In addition, most etch applications of high density plasma etchers use two sources of electrical power, one to generate the plasma (e.g., source power) and one to bias the wafer (e.g., bias power). An exemplary etcher in which processing can take place is a Lam 9100 Etcher, which is a dual source high density plasma etcher.

[0043] For example, in using a dual source high density plasma etcher (not shown) in a semiconductor processing method to form an etched feature (e.g., contact opening) according to the invention, the etcher is configured at a first bias power and a first source power, and a substrate 12 having a masked layer 14 formed thereover through which a contact opening 22 is to be etched, is placed onto the wafer receiver of the etcher. The etchable layer 14 is plasma etched while the etcher is powered at the first bias power and the first source power, to form the feature 22 to a first selected depth D₁, preferably a feature 22 having an aspect ratio of at least about 0.5 μm.

[0044] The etcher is then set at a selected deposition time, and configured to vary the source power and vary the bias power, whereupon an etch resistant polymer material is deposited onto the etchable layer 14 by PECVD utilizing a polymerizing gas to form the self-aligned mask 30 over the surface 28 of the masking layer 16 and a portion of the sidewalls 24 contiguous with the surface 28. Substantially no polymeric material is deposited onto the etchable layer at the bottom portion 26 of the opening 22 during the deposition step. The etcher is then configured to vary the source power and vary the bias power, and the etchable layer 14 is plasma etched to extend the feature 22 downwardly to a second selected depth D₂, resulting in the structure shown in FIG. 1D.

[0045] To form or deposit a self-aligned mask layer 30 about 3000 angstroms thick over the surface 28 of a masking layer 16 and a portion of the sidewalls 24, and form a breadloaf area 34 about 700 angstroms thick over the comers 32, using an Applied Materials IPS Etcher and one or more fluorocarbon gases, the following settings can be used:

[0046] Source (top) Power: From about 800 to about 1400 watts, preferably about 1100 watts

[0047] Bias (bottom) Power: From about 1 to about 200 watts, preferably about 100 watts

[0048] Pressure: From about 15 to about 40 mtorr, preferably about 20 mtorr

[0049] Duration: From about 10 to about 60 seconds

[0050] Flow Rate (C₄F₈and CHF₃): From about 5 to about 25 sccm (each)

[0051] There is an interaction between the above listed parameters, and the settings can be altered to customize the deposition rate, formation and the thickness of the self-aligned mask layer 30 over the etchable layer 14. For example, during the deposition step, the source power, the bias power and/or the deposition time can be varied to control and modify the thickness of the self-aligned mask such that an adequate mask thickness is provided to prevent etching of the surface, comers and sidewalls of the layer, yet maintain a feature having a width sufficient to ensure the desired etch results. Preferably, the deposition of the polymer is regulated to control the extent and thickness of a breadloaf formation 34 such that the mouth 36 of the feature 22 is not substantially obstructed which would hinder the subsequent etch of the feature downwardly into the etchable layer 14 and result in the profile of the sidewalls becoming tapered rather than being etched vertical.

[0052] For example, forming a relatively thin layer of the self-aligned mask layer 30 of about 1000 to about 1500 angstroms/microns on the surface 28 of the masking layer 16 and sidewalls 24 results, at least in part, from operating at a relatively high bias voltage of about 50 to about 150 watts, and a lower deposition rate of about 1000 angstroms/minute, which would include lowering the source power to the etch apparatus to about 800 to about 900 watts. The thickness T₁of the self-aligned mask layer 32 can also be varied by the duration of the deposition step. After a short transient period, the layer thickens as the deposition time is increased. Therefore, by limiting the duration of the deposition process, a relatively thin layer of the self-aligned mask layer 32 can be deposited.

[0053] In general, the selectivity of the self-aligned mask material in the subsequent etch process depends on the chemical structure of the film. The fluorocarbon polymer chemistry can be used as an example. It is well known that films that have a higher carbon/fluorine atomic ratio usually have lower etch rates than films with lower carbon/fluorine ratios. Methods of increasing this ratio include higher source power to dissociate more free fluorine from the feed gas, or higher bias power to sputter more fluorine from the polymer film. The interaction between the flow rate, powers, and other chamber variables makes it difficult to quantify how the selectivity will change with conditions. The optimization of the process involves obtaining the desired self-aligned shape of the deposit and then varying conditions inside the shape-defined process window to obtain adequate selectivity. Feed gases that are relatively fluorine deficient, such as C₄F₈, are generally preferred if selectivity is an issue.

[0054] Upon depositing a sufficiently thick layer 30 of the self-aligned mask over the surface 28 of the masking layer 16 and, preferably, over the corners 32 and onto the adjoining portion of the sidewalls 24 of the feature 22, the feature 22 is further etched downwardly into the etchable layer 14, as shown in FIG. 1D. Etching of the feature 22 can be according to a conventional etch process known and used in the art, preferably by plasma etch as discussed hereinabove for the first etching of the contact opening.

[0055] Although not shown, upon etching the contact opening 22 to the desired depth D₂, the self-aligned mask layer 30 and remaining masking layer can be removed to uncover the etchable layer 14, corners 32a, and the sidewalls 24 of the feature 22. A conventional removal process similar to those used for stripping photoresist, for example, a conventional plasma asher, can typically be used to remove the self-aligned mask layer 30 and masking layer 16. The removal step is performed for a time duration effective to remove the polymer material and masking material overlying the etchable layer 14, which is determined, at least in part, by the thickness of the layers 16, 30.

[0056] Optionally, after removing the self-aligned mask layer 30, the substrate 12 at the bottom portion 26 of the feature 22 can be cleaned to remove contaminants including hydrocarbons, fluorocarbons, and other organic residues. A conventional O₂clean etch can be used to remove organic residue from the substrate 12 at the bottom portion 26 of the feature.

[0057] Although not shown, the feature 22 can be further extended downwardly into the etchable layer 14 or a layer of material (not shown) underlying the etchable layer 14, to a predetermined depth D₃, by repeating the foregoing steps of depositing the self-aligned mask layer 30 over the layer to be etched, and further etching the feature 22 to a desired depth.

[0058] In another embodiment, depicted in FIGS. 2A-2C, the method can be used to form a self-aligned mask over a patterned masking layer having one or more openings, to build up and/or protect the masking layer for etching a feature onto a substrate. A wafer 10′ is depicted in FIG. 2A as having a photoresist or other masking layer 16′ deposited on a layer 14′ to be etched to form a desired feature 22′. The masking layer 16′ has been deposited to a predetermined thickness to provide the opening 18′ formed in the masking layer with a sufficient depth D_asuch that polymer material subsequently deposited onto the wafer 10′ to form the self-aligned mask is not substantially deposited onto the etchable layer 14 at the bottom portion 26′ of the opening 18′. The self-aligned mask layer 30′ is then deposited from a polymerizing gas, as previously described, onto the surface 28′ of the masking layer 16′ and into the opening 18′. The feature 22′ can then be etched downwardly into the etchable layer 14′ to a desired depth D₁. As previously described, additional layers of the self-aligned mask can be deposited over the existing mask layers 16′, 30′, to continue the etch of the feature 22′.

[0059] In certain applications, a layer of material is deposited into a feature such as a contact opening and it is desirable to maintain the deposited layer on the sidewalls but not the base of the opening. According to another embodiment of the method of the invention, a self-aligned mask layer can be used to selectively remove an etchable material from the bottom portion of a contact opening or other feature, as depicted in FIGS. 3A-3D.

[0060] Referring to FIG. 3A, the wafer fragment 10″ includes a semiconductive substrate 12″ and a first layer of an etchable material 14″ formed over the substrate 12″ into which a feature 22″ (e.g., contact opening) has been formed by a conventional prior art etch process. A second layer of an etchable material 40 has been deposited onto the first etchable layer 14″, including the sidewalls 24″ and bottom portion 26″ of the feature 22″.

[0061] Exemplary materials for the second etchable layer 40 include silicon nitride (Si₃N₄), silicon oxides such as silicon dioxide, oxide deposited from tetraethylorthosilicate (TEOS), or a doped silicon dioxide such as phosphosilicate glass (PSG) and borophosphosilicate glass (BPSG), a conducting material such as polysilicon or a metal, or a seed layer to control, for example, subsequent selective depositions. Such materials can be deposited onto the wafer by conventional deposition methods as known and used in the art, for example, by a low pressure chemical vapor deposition (LPCVD) process.

[0062] According to the invention, a layer of polymer material that is resistant to the etch gas mixture of a subsequent etch step is deposited surface-wide over the wafer surface, to result in the structure shown in FIG. 3B. The polymer material is deposited as a layer to form a self-aligned mask layer 30″ over the second etchable layer 40. Preferably, the self-aligned mask layer 30″ is formed over both the surface 42 of the second etchable layer 40 and extends continuously and conformally over the corners 44 and into the feature 22″ to cover a portion of the second etchable layer 40 overlying the sidewalls 24″. It is preferred that a breadloaf area 34″ is formed over the corners 44 having a thickness effective to prevent etch-off or faceting of the corners 44 and lateral etching of the second etchable layer 40 overlying the sidewalls 24″, but not substantially obstruct the mouth 36″ of the feature 22″ during a subsequent etch of the feature 22″ to remove the portion of the second etchable layer 40 overlying the bottom portion 26″ of the feature 22″.

[0063] The self-aligned mask layer 30″ can be deposited to a desired thickness T₁as described hereinabove with regard to the first embodiment of the method, using one or more of the aforementioned polymerizing gases in a conventional PECVD process or other suitable deposition technique. In general, a mask layer 30″ having a thickness T₁of about 200 to about 1000 angstroms is suitable for most applications. The optimal T₁value depends on the thickness of the film 40, the aspect ratio of the feature 22″ and the selectivity between the film 40 and the self- aligned mask layer 30″ during the etch that removes the second etchable layer 40. As previously discussed, the thickness and selectivity of the self-aligned mask layer 30″ can be controlled and modified by varying the process settings or configuration of the plasma etcher used to deposit the polymer material onto the wafer 10″.

[0064] To ensure the selective removal of the second etchable layer 40 from the bottom portion 26″ of the feature 22″, it is preferred that substantially no polymerizing gas is deposited onto the second etchable layer 40 overlying the base 26″ of the feature 22″ during the formation of the self-aligned mask layer 30″. To control this, at least in part, the feature 22″ is etched to a depth (D″) whereby the aspect ratio is at least about 0.5, preferably at least about 1. Deposition of the polymerizing gas into the feature 22″ can also be controlled by varying the parameters used in applying the polymerizing gas, as discussed hereinabove.

[0065] Referring to FIG. 3C, upon forming the self-aligned mask layer 30″ having the desired thickness T₁and coverage of the second etchable layer 40, the wafer 10″ is then exposed to an etch gas using a conventional etch removal technique, to selectively etch at least a portion of the second etchable layer 40 from the bottom portion 26″ of the feature 22″. For example, an etch gas comprising hydrofluocarbons, such as CHF₃or CH₂F₂, can be used to selectively etch a layer 40 of Si₃N₄from bottom portion 26″ comprising a silicon material (substrate 12″). By further example, an etch gas comprising Cl₂or HBr, can be used to selectively etch a layer 40 of polysilicon from bottom portion 26″ comprising a silicon material (substrate 12″). For a layer 40 that has properties that are similar to the bottom portion or base 26″, for example, polysilicon layer 40 and crystalline silicon layer 12″, an etch stop layer such as a thin oxide is typically formed on the base prior to the polysilicon deposition.

[0066] Although not shown, the feature 22″ can be further extended past the etchable layer (e.g., into substrate 12″) to form a feature having sidewalls made of two differing materials. The feature 22″ can also be etched to a desired depth by employing repetitive deposits of additional mask layers onto the existing mask layer and etching the feature downwardly (not shown).

[0067] After completion of the etch to remove the second etchable layer 40 selectively from the bottom portion 26″ of the feature 22″, the self-aligned mask layer 30″ can be removed as desired to expose the second etchable layer 40 including the surface 42 and the corners 46, and the sidewalls 24″ of the feature 22″ to result in the structure shown in FIG. 3D. A conventional removal process similar to those used for stripping photoresist as discussed hereinabove, can be used to remove the self-aligned mask layer 30″, preferably by removing the layer with an oxygen-containing plasma—standard ashing, by use of a plasma stripper/plasma asher. The duration of the removal step depends, at least in part, on the thickness of the self-aligned mask layer 30″. Optionally, the substrate 12′ at the bottom portion 26″ of the feature 22″ can be surface treated to remove organic residues, for example, by a conventional O₂clean etch.

[0068] The following example is given to illustrate specific embodiments of the present invention. Numerous other variations, however, are within the scope of the present invention.

[0069] A partially etched opening 8000 angstroms deep was extended to 1.8 μm deep using the self-aligned mask of the invention. This corresponds to extending the aspect ratio from approximately 3 to greater than 7 with the self-aligned mask as the only mask used.

[0070] An Applied Materials IPS etcher was used to plasma etch the substrate to form an opening using a CHF₃plasma (870 W source power, 800 W bias power, 22 sccm CHF₃, 20 mtorr pressure, 200° C. ring temperature, 140° C. roof temperature), and to deposit the self-aligned mask by PECVD in the same chamber (1044 W source power, 100 W bias power, 10 sccm C₄F₈, 10 sccm CHF₃, 20 mtorr pressure, 200° C. ring temperature, 140° C. roof temperature).

[0071] As shown in FIG. 4A, a first BPSG substrate was etched to a depth of about 8000 angstroms to form an initial contact opening (870 W source power, 800 W bias power, 22 sccm CHF₃, 20 mtorr pressure, 200° C. ring temperature, 140° C. roof temperature). The original photoresist layer was then ashed, and a self-aligned film was deposited by PECVD (1044 W source power, 100 W bias power, 10 sccm C₄F₈, 10 sccm CHF₃, 20 mtorr pressure, 200° C. ring temperature, 140° C. roof temperature, 60 seconds). The resulting structure had significant breadloafing about the mouth of the opening. The deposition of the self-aligned mask film did not etch the oxide.

[0072] As shown in FIG. 4B, the BPSG layer of FIG. 4A was then etched (870 W source power, 800 W bias power, 22 sccm CHF₃, 20 mtorr pressure, 200° C. ring temperature, 140° C. roof temperature)). The etch extended the depth of the opening by 6000 angstroms, and the top surface dimensions remained unchanged. The width dimension of the contact opening was the same as in FIG. 4A. The deposited self-aligned mask layer continued to protect the top surface of the feature during the etch.

[0073] A second BPSG substrate was etched using a self-aligned mask using two deposition/etch cycles, according to the same conditions and settings as in FIGS. 4A-4B. As shown in FIG. 4C, an opening in an oxide layer of BPSG was first etched to a depth of 6000 angstroms using a photoresist mask. The photoresist was removed, and a self-aligned mask layer was deposited onto the surface of the BPSG layer by PECVD. The substrate was then processed through two deposition/etch cycles using the self-aligned mask layer. The process extended the depth to 1.4 μm. The resulting opening was 0.6 μm deeper than the opening provided by the previously described single deposition/etch cycle.

[0074] FIG. 4D shows the result of processing a third BPSG substrate through four deposition/etch cycles, according to the conditions and settings of FIGS. 4A-4B, using the self-aligned mask layer. Using a photoresist mask, the BPSG substrate was initially etched to a depth of 0.8 μm. The photoresist mask was removed, and the substrate was subjected to four deposition/etch cycles, resulting in an extension of the depth of the contact opening by 1 μm. The ratio of deposition/etch was high, as evidenced by the thick deposit of the self-aligned mask layer on the surface of the BPSG layer.

[0075] The result shown in FIG. 4A-4D demonstrate the feasibility of the process in a number of specific areas. It is possible to deposit the self-aligned mask layer on an etched feature that has been cleaned of resist, and to use the layer as an etch mask to deepen the feature. It is also shown that multiple applications of the deposit and etch can b used. This is critical for profile control and total etch depth. For example, the breadloafing seen in FIG. 4A occurred in the deposition cycles in FIG. 4D (the same deposition was used in all of these examples). It is apparent that a shorter deposition time would result in less breadloafing and a less tapered profile in FIG. 4D. Thus, the examples demonstrate the performance of the invention and suggest the proper optimization scheme to tailor the profiles to meet the requirements of the process. In addition, the removal of the photoresist mask after the first etch was done to evaluate this particular embodiment of the invention. The removal of this mask is not necessary. If the remaining resist mask is not removed from the surface, then thinner self-aligned layers are required to achieve the same result. Both methods can be used, depending on the requirements of the process.

[0076] In compliance with the statute, the invention has been described in language more or less specific as to structural and methodical features. It is to be understood, however, that the invention is not limited to the specific features shown and described, since the means herein disclosed comprise preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims appropriately interpreted in accordance with the doctrine of equivalents.