Cyclic directional / isotropic etch

Abstract

Examples are disclosed that relate to controlling a width of a feature as a function of depth when etching the feature. One example provides a method of etching a feature in a substrate. The method comprises performing a plurality of etching cycles, each etching cycle comprising performing a directional etch step in which a liner is formed along a sidewall of the feature using a liner-forming substance, and performing an isotropic etch step to widen the feature at an etch front of the feature.

Description

CYCLIC DIRECTIONAL / ISOTROPIC ETCHBACKGROUND

[0001] Electronic device fabrication processes can involve many steps of material deposition, patterning, and removal to form integrated circuits on substrates. Some integrated circuit processes can involve etching high aspect ratio (HAR) features. A HAR feature is a feature with a much greater depth than width. HAR features are sometimes formed in the fabrication of three-dimensional integrated circuits, in which device layers are stacked onto one another to increase a number of devices per unit area of a substrate.SUMMARY

[0002] This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. Furthermore, the claimed subject matter is not limited to implementations that solve any or all disadvantages noted in any part of this disclosure.

[0003] Examples are disclosed that relate to controlling a width of a feature as a function of depth when etching the feature. One example provides a method of etching a feature in a substrate. The method comprises performing a plurality of etching cycles, each etching cycle comprising performing a directional etch step in which a liner is formed along a sidewall of the feature using a liner-forming substance, and performing an isotropic etch step to widen the feature at an etch front of the feature.

[0004] In some such examples, performing the directional etch comprises forming a plasma using a gas mixture comprising one or more of a chlorine-containing etchant or a bromine-containing etchant, and wherein the gas mixture also comprises an oxygen-containing liner-forming substance.

[0005] Alternatively or additionally, in some such examples, the substrate comprises a stack of alternating layers of silicon and silicon-germanium.

[0006] Alternatively or additionally, in some such examples, the isotropic etch step omits the oxygen-containing liner-forming substance.

[0007] Alternatively or additionally, in some such examples, performing the directional etch step comprises applying a bias to accelerate formed in a plasma toward the substrate, and wherein performing the isotropic etch step comprises not applying the bias.

[0008] Alternatively or additionally, in some such examples, the substrate comprises a stack of alternating layers of silicon nitride and silicon oxide, and wherein performing the directional etch step comprises using a fluorine-containing etchant and a carbon-containing liner-forming substance.

[0009] Alternatively or additionally, in some such examples, the substrate comprises a stack of alternating layers of silicon oxide and polycrystalline silicon, and wherein performing the directional etch step comprises using a fluorine-containing etchant and a carbon-containing liner-forming substance.

[0010] Alternatively or additionally, in some such examples, performing the directional etch step comprises forming a transformer coupled plasma (TCP).

[0011] Alternatively or additionally, in some such examples, the isotropic etch step is a plasma-enhanced etch step.

[0012] Alternatively or additionally, in some such examples, the isotropic etch step is a thermal etch step.

[0013] Alternatively or additionally, in some such examples, the feature has a substantially uniform width as a function of depth.

[0014] Alternatively or additionally, in some such examples, the feature is a reentrant feature.

[0015] Another example provides a method of etching a feature in a substrate comprising a stack of alternating layers of silicon and silicon / germanium. The method comprises performing a cyclic directional etch / isotropic etch process for a plurality of cycles, each cycle comprising performing a directional etch step using a halogencontaining etchant and an oxygen-containing liner-forming substance to increase a depth of the feature; and performing an isotropic etch step comprising a chlorine radical-based etching process that widens the feature at an etch front.

[0016] In some such examples, the method is used in fabrication of a three dimensional (3D) vertical dynamic random access (DRAM) memory fabrication process.

[0017] Another example provides a processing tool. The processing tool comprises a processing chamber, a substrate support disposed in the processingchamber, a remote plasma chamber, a radiofrequency power source configured to form a plasma in the remote plasma chamber, a biasing power source configured to accelerate ions created in the remote plasma chamber toward the substrate, flow control hardware, and a controller. The controller is configured to perform a plurality of etching cycles, at least one etching cycle comprising a directional etch step and an isotropic etch step. The controller is configured to, in the directional etch step, operate the radiofrequency power source, the bias power source, and the flow control hardware to form a plasma comprising at least an etchant and a liner-forming substance to etch a feature of a substrate and deposit a liner along a sidewall of the feature. The controller is further configured to, in the isotropic etch step, operate the radiofrequency power source to form a plasma using at least an etchant that is selective to the liner to isotropically etch the substrate at an etch front of the feature.

[0018] In some such examples, the substrate comprises a stack of alternating layers of silicon nitride and silicon oxide, and the controller is configured to, in the directional etch step, form a plasma comprising a fluorine-containing etchant and a carbon-containing liner-forming substance.

[0019] Alternatively or additionally, in some such examples, the substrate comprises a stack of alternating layers of silicon oxide and polycrystalline silicon, and the controller is configured to, in the directional etch step, form a plasma comprising a fluorine-containing etchant and a carbon-containing liner-forming substance.

[0020] Alternatively or additionally, in some such examples, the substrate comprises a stack of alternating layers of silicon and silicon-germanium, and the controller is configured to, in the directional etch step, form a plasma comprising an oxygen-containing liner-forming substance and one or more of a chlorine-containing etchant or a bromine-containing etchant.

[0021] Alternatively or additionally, in some such examples, the controller is configured to, in the isotropic etch step, form a plasma using chlorine.

[0022] Alternatively or additionally, in some such examples, the controller is configured to perform the plurality of etching cycles to form a high aspect ratio feature in the substrate.BRIEF DESCRIPTION OF THE DRAWINGS

[0023] FIG. 1 schematically shows an example etching process in which a relatively stronger passivation used to protect sidewalls of a feature lead to tapering of a feature as a function of feature depth.

[0024] FIG. 2 schematically shows an example etching process in which a relatively weaker passivation used to protect sidewalls of a feature lead to bowing (widening) near the top of the feature.

[0025] FIG. 3 shows a flow diagram of an example method for etching a feature using a cyclical directional etch / isotropic etch process.

[0026] FIGS. 4A-4B schematically show evolution over time of an example feature being etched using the process of FIG. 3.

[0027] FIGS. 5A-5B show measured critical dimension as a function of depth for features etched by directional etch processes that include or omit a cyclical isotropic etch step.

[0028] FIG. 6 schematically shows an example processing tool that can be used to etch a feature in a substrate.

[0029] FIG. 7 shows a block diagram of an example computing system.DETAILED DESCRIPTION

[0030] The term “aspect ratio” generally represents a ratio between a depth of a substrate feature such as a hole and an average width of the feature. The term high aspect ratio (HAR) represents an aspect ratio of 5:1 or greater.

[0031] The term “critical dimension” generally represents a width of a feature, such as a diameter of a hole.

[0032] The term “etch” and variants thereof generally represent a process in which a material is selectively removed from a substrate. An etch using gas phase etchants is referred to as a “dry etch”.

[0033] The term “directional etch” generally represents an anisotropic etch that proceeds at different rates in different directions. For example, a directional etch can be used to etch vertically (normal to a plane of a substrate surface) at a higher rate than horizontally (parallel to a plane of a substrate surface).

[0034] The term “isotropic etch” generally represents an etch that proceeds at substantially similar rates in different directions.

[0035] The term “etchant” generally represents a chemical used in an etching process to chemically remove and / or facilitate chemical removal of material from a substrate. The term “etchant gas mixture” generally represents a mixture of one or more gases comprising at least one etchant.

[0036] The term “feature” generally represents topology on a substrate surface. A feature can be formed by etching.

[0037] The term “liner” generally represents a protective layer formed on sidewalls of a feature being etched. A liner can help to prevent widening of the feature when performing a directional etch.

[0038] The term “liner-forming substance” generally represents a chemical substrate that reacts with etched substrate material to form a liner on the surface of the substrate.

[0039] The term “processing chamber” generally represents an enclosure in which chemical and / or physical processes are performed on substrates. The pressure, temperature and atmospheric composition within a processing chamber may be controllable to perform chemical and / or physical processes.

[0040] The term “processing tool” generally represents a machine including a processing chamber and other hardware configured to enable processing to be carried out in the processing chamber.

[0041] The term “plasma” generally represents a gas comprising ions and free electrons.

[0042] The term “transformer coupled plasma” generally represents a plasma in which energy that forms the plasma is provided by time-varying magnetic fields using a coil.

[0043] The term “substrate” generally represents any object that can be processed in a processing chamber of a processing tool. Deposition and etching are example processes that can be performed on a substrate in a processing chamber.

[0044] The term “substrate support” generally represents any structure for supporting a substrate in a processing chamber.

[0045] As mentioned above, some integrated circuit fabrication processes involve etching a high aspect ratio (HAR) feature into one or more previously deposited material layers. Some HAR features can have aspect ratios on the order of microns by tens of nanometers. HAR features are often formed in the fabrication of vertical or three-dimensional (3D) integrated circuits. Examples include 3D NAND (NOT AND)memory devices and vertical or 3D dynamic random access memory (VDRAM or 3D DRAM). 3D integrated circuits include multiple device layers, in which layers of devices are stacked upon one another to increase a density of circuit structures on a substrate.

[0046] The fabrication of 3D circuits can involve depositing stacks of alternating layers of materials, and then etching HAR features through the alternating stacks. Then, further processing is used to form circuit structures (e.g. memory cells) within the holes. As more specific examples, VDRAM fabrication can involve the deposition of alternating layers of silicon (Si) and silicon-germanium (SiGe), followed by directional (vertical) etching to form HAR features in the Si / SiGe stack. Similarly, 3D NAND fabrication can involve the deposition of alternating layers of silicon oxide and silicon nitride, or alternating layers of silicon oxide and polycrystalline silicon (poly silicon), followed by directional etching of features through the alternating layers.

[0047] The directional etching processes used to form such HAR features (e.g. reactive ion etching (RIE)) involve forming ions from etchants in a plasma, and accelerating the ions toward a substrate using an applied bias. A patterned hard mask can be used to define the locations and geometries of features being etched. The hard mask material is chosen based upon the etching chemistry used to form the feature to achieve a suitably high etch selectivity between the hard mask and the material (or materials) being etched.

[0048] As mentioned above, the aspect ratio of some HAR features can be on the order of 10s to 100s. Proper functioning of the circuit structures throughout the vertical dimension of the 3D circuits, and therefore device yield, can be dependent upon achieving a suitably uniform width of a feature throughout the depth of the feature. However, even with highly directional etching processes, the ions accelerated toward the substrate have an angular velocity distribution. As such, some ions can impact a sidewall of a feature being etched, and thereby damage the sidewall. This can result in the feature being wider than desired closer to an opening of the feature, where the sidewalls are more exposed to ions having non-vertical directions.

[0049] One method of protecting the sidewall closer to an opening of a feature being etched is to use a relatively thicker hard mask to define the features being etched. The use of a suitably thick hard mask can result in the non-vertical ions primary impacting the sidewall of the hard mask, and not the underlying material being etched. However, even a highly selective hard mask material can be eroded during an etchingprocess. As the hard mask material is eroded, more non-vertical ions can impinge the sidewall of feature within the material being etched, thereby widening the feature in the affected regions.

[0050] Alternatively or additionally to using a relatively thicker hard mask, the etching chemistry can include a liner-forming substance that reacts with etching byproducts to form a liner on the surface of the substrate. As an example, when etching a HAR feature into an Si / SiGe stack, a plasma containing one or more halogencontaining etchants and also an oxygen-containing liner-forming substance can be used. The one or more halogen-containing etchants can form volatile halides (e.g. SiX4 and GeX4, where X represents any suitable halogen atoms or combination of halogen atoms). Such volatile halides can redeposit as byproducts on surfaces within the HAR feature.

[0051] The oxygen-containing liner-forming substance can form reactive oxygen-containing species that oxidize the silicon atoms in these byproducts to form a silicon oxide (SiCh) liner. Silicon oxide is highly selective with regard to the halogenbased etching chemistries used to etch Si and SiGe. This is because silicon oxide is more thermodynamically stable than the silicon halides formed in the etching process. The silicon oxide liner can be eroded at the bottom of the feature being etched due to the highly directional, energetic ion flux at the bottom of the feature being etched. This allows vertical etching to continue. Further, the silicon oxide liner formed on the sidewall can help prevent the widening of a feature as the etching progresses, as the sidewall experience a lower ion flux than the bottom of the feature at the etch front, and thus can protect the sidewall. Further, a sufficiently thick silicon oxide liner can help protect a region of the sidewall of the feature at and just below the opening of the feature as angular ion flux experienced by this sidewall region increases due to hard mask erosion. Carbon may similarly be used as a liner when etching a silicon oxide / silicon nitride stack, or a silicon oxide / polysilicon stack, in a 3D NAND fabrication process.

[0052] However, the use of a liner to protect a sidewall from etching during a directional etching process can pose challenges. For example, the use of too low a concentration of the liner-forming substance can form too thin of a liner. This can allow ions with non-vertical directions to break through the liner and widen the feature in a region just below a hard mask, where an angular ion flux experienced by the sidewall is highest. This can be referred to as “bowing”. The resulting sidewall profile can be referred to as “bow.” On the other hand, the use of too high a concentration of liner canresult in the formation of a feature with a tapered width that narrows as a function of increasing depth. More particularly, as a thickness of a liner increase over time due to additional byproduct production and oxidation, the possibility of byproduct remaining at the etch front increases. The byproduct remaining at the HAR feature etch front can lead to a tapered trench profile as the etching process continues, as the byproduct at the etch front can increase a thickness of the liner on the sidewalls at the etch front. Thus, forming a HAR feature having a suitably uniform width throughout the depth of the HAR feature can pose a challenging balance between avoiding taper from too much liner-forming substance, and avoiding bow from too little liner-forming substance.

[0053] FIG. 1 shows an example substrate 100 in which a tapered HAR feature 102 is etched. In the example of FIG. 1, a patterned hard mask 104 is disposed on the substrate 100 to define a width of the HAR feature 102. The patterned hard mask 104 has been eroded by the etching process. An original boundary of the patterned hard mask is shown in dashed lines 104A.

[0054] In some examples, the substrate can include silicon or a stack of Si / SiGe layers, and can be etched using a halogen-based etching process. As more detailed examples, the etching chemistry can include one or more of hydrogen bromide (HBr), hydrogen chloride (HC1), bromine (Bn), chlorine (Ch), an interhalogen such as BrCl and / or other suitable interhalogen to etch the silicon and SiGe. Further, an oxygencontaining substance can be used as a liner-forming substance to form a silicon oxide liner 106. Examples of oxygen containing substances include oxygen gas (Ch), ozone (O3), water vapor (H2), hydrogen peroxide (H2O2), nitrous oxide (N2O), other nitrogen oxides, and carbon oxide (e.g. carbon dioxide (CO2) and carbon monoxide (CO)). In such examples, the hard mask can be silicon oxide, silicon oxynitride, or other suitably selective material to the halogen-based etching chemistry used.

[0055] In the example of FIG. 1, a relatively higher concentration of the linerforming substance was used to form the liner 106. This can help to avoid bowing of the sidewall of the HAR feature 102 in the region immediately below the hard mask 104, where the sidewall of the HAR feature 102 is exposed to more angular ion flux. However, the liner 106 also causes a tapered narrowing of the HAR feature 102 as a function of increasing depth. Such tapering is shown in FIG. 1 where a feature width near the etch front at 108 is narrower than a feature width at the top of the feature at 110. As mentioned above, this can be due to oxidizing more etching byproducts at the etch front, thereby impeding widening of the feature at the etch front. The resultingtapered profile can impact device performance, and thereby potentially impact device yields.

[0056] To avoid forming such a narrowing taper, a relatively lower concentration of the liner-forming substance can be used. FIG. 2 shows another substrate 200 in which a HAR feature 202 is being etched, and on which a liner 206 has been formed. Like substrate 100, the substrate 200 comprises a patterned hard mask 204 that has been eroded by the etching process. An original boundary of the patterned hard mask is shown in dashed lines 204A. The erosion of the hard mask 204 can expose the sidewalls of the HAR feature 202 to angular ion flux. In this example, because a relatively lower concentration of the liner-forming substance was used, the liner 206 has been eroded in the region immediately below the hard mask 204, causing bowing of the HAR feature 202 in the region below the hard mask (indicated at 208). Again, this bowing can impact device yields.

[0057] Accordingly, examples are disclosed that relate to forming HAR features with suitably uniform widths in manners that address the issues described above. Briefly, the disclosed examples utilize a cyclic process in which directional etch steps are interspersed with isotropic etch steps. The directional etch steps are performed using an etching chemistry that includes a liner-forming substance. The directional etch steps utilize a plasma (e.g. a transformer coupled plasma (TCP) or a capacitively coupled plasma (CCP)) to form reactive ions from the etching chemistry. As a more detailed example, to etch a HAR feature in a substrate comprising a stack of Si / SiGe layers, the substrate chemistry can include one or more halogen etchants, and an oxygen-containing liner-forming substance. Reactive ions from the halogen etchants are accelerated toward the substrate to etch the feature in substrate regions not protected by a hard mask. Further, reactive ions from the oxygen-containing liner-forming substance react with silicon halide byproducts of the etching process to form a liner on sidewalls of the feature.

[0058] As mentioned above, the use of too low a concentration of the linerforming substance in the etching chemistry can result in widening of the feature (“bowing”) in the area immediately below the hard mask. Thus, a sufficient concentration of the liner-forming substance is used to avoid such bowing. Further, to avoid forming a HAR feature with a tapered width that narrows as a function of increasing depth, an isotropic etch step is performed one or more times, interspersed with directional etch steps, during the HAR feature etching process. The isotropic etchstep can use a similar or a same etching chemistry as the directional etch step, but omit the liner-forming substance. The isotropic etch also is performed at a lower bias than the directional etch. In some examples, the bias used to accelerate ions toward the substrate during the directional etch steps is turned off during the isotropic etch steps.

[0059] The isotropic etch step can be a plasma-enhanced isotropic etch step, or a thermal isotropic etch step. In a plasma-enhanced isotropic etch step, reactive species are formed from etchants in a gas mix introduced into a plasma. The reactive species include ions and radicals formed form the etchants in the plasma. However, in the absence of a bias (or in the presence of an insufficient bias) to cause directional etching, few ions formed in the plasma reach the etching front at the bottom of the feature. This can be due, for example, to recombination of ions to form neutral, less reactive species. On the other hand, radicals formed in the plasma can have a longer lifetime than the ions. Thus, the radicals formed from the etchants in the plasma can diffuse to the bottom of the feature. In a thermal etch step, thermal energy is used to activate the etchants at the substrate.

[0060] As mentioned above, the liner formed during the directional etch step is selected to be more thermodynamically stable than the volatile products formed during etching. As an example, a silicon oxide liner formed on silicon or SiGe is more thermodynamically stable than volatile silicon halides formed during etching. Further, the reactive halogen-containing radicals formed in the plasma during the isotropic etch step lack sufficient kinetic energy to erode the liner. Additionally, the formation of the silicon oxide liner is an accumulative process and not uniform from trend top to bottom. Instead, the liner tends to be thicker farther from the etch front, and thinner closer to the etch front. The etch front itself can have minimal liner. Thus, the halogen-containing radicals formed in the plasma can etch silicon and / or SiGe at the etch front, where the liner is very thin or nonexistent, but not at other locations closer to the opening of the feature. This selectively widens the feature at the etch front, without widening the feature at locations closer to the feature opening than the etch front. In this manner, the feature can intermittently be widened at the etch front to avoid taper, between directional etch steps that increase the feature depth.

[0061] FIG. 3 shows a flow diagram depicting an example method 300 for forming etching a feature, such as an HAR feature, by performing directional etching and isotropic etching in a cyclical manner. Method 300 comprises, at step 302, performing a directional etch step in which a liner is formed on sidewalls of feature. Inthe directional etch step, a gas mixture comprising at least an etchant and a linerforming substance is introduced into a plasma. A bias is applied to accelerate ions formed in the plasma toward the substrate, thereby leading to etching of the substrate and liner deposition. Step 302 can be performed for various different materials systems. As one example, and referring to box 304 of FIG. 3, a silicon layer, or a stack of alternating silicon / SiGe layers, can be directionally etched using one or more halogencontaining etchants. In such a materials system, a silicon oxide can be formed on sidewalls of the feature being etched by including an oxygen-containing liner-forming substance in the etching chemistry. Examples of halogen-containing etchants and oxygen-containing liner-forming agents include those described above.

[0062] As another example materials system, a feature can be directionally etched into a stack of alternating silicon oxide / silicon nitride layers using a fluorine- containing etchant. Example fluorine-containing etchants include hydrogen fluoride (HF), nitrogen trifluoride (NF3), sulfur tetrafluoride (SF4), sulfur hexafluoride (SFe), fluorocarbons (materials having a molecular formula CxFy, where x can be 1 or more and y can vary depending upon the number of carbon atoms and the nature of carboncarbon bonding in the molecule) and hydrofluorocarbons (materials having a molecular formula CxFyHz, where x can be 1 or more, and y and z can vary depending upon the number of carbon atoms, the nature of carbon-carbon bonding in the molecule, and a ratio of fluorine to hydrogen atoms in the molecule) that can be volatilized for introduction into a plasma. In some examples, an etchant gas mixture can comprise a fluorine-containing etchant and one or more of H2O, an alcohol, or pyridine.

[0063] Further, in the example of etching a HAR feature into a stack of alternating silicon oxide / silicon nitride layers, a carbon-containing substance can be used as a liner-producing substance to produce a carbon-containing liner in the feature being etched. An example carbon-containing liner is amorphous carbon. Amorphous carbon is highly selective to fluorine-containing etchants that can be used to etch silicon oxide and silicon nitride. Example carbon-containing substances that can be used to form an amorphous carbon liner include alkanes having a general formula CnH2n+2 where n is an integer in a range of 1 to 10 (such as, methane, ethane, etc.), alkenes having a general formula CnH2n where n = 2 to 10 (such as, ethylene, propylene, etc.), and alkynes having a general formula CnH2n-2 where n = 2 to 10 (such as, acetylene, propyne, etc.), that are gas-phase under processing conditions. Other examples of carbon-containing substances that can be used to form an amorphous carbon liner caninclude aromatic hydrocarbons, cyclic aliphatic hydrocarbons, heterocyclic compounds, and alkyl amines and other nitrogen-containing compounds, that are gasphase under processing conditions and that include carbon-containing functional groups.

[0064] As a further example of a materials system in which a HAR feature can be etched according to the present disclosure, a stack of alternating layer of silicon (e.g. polysilicon) and silicon oxide can be etched using a fluorine-containing etchant, and a carbon-containing substance can be used as a liner-forming substance.

[0065] Continuing with FIG. 3, the method 300 further comprises, at 306, performing an isotropic etch step. In some examples, in the isotropic etch step of FIG. 3, reactive etching species are formed from an etchant in a plasma, but no bias is applied to accelerate ions toward the etch front. This results in a radical-based etch at the etch front. In other examples, thermal energy is used to activate the etchant at the etch front.

[0066] At the etch front, the liner can be absent, or can be sufficiently thin to allow the etching to occur, even where the isotropic etching chemistry is highly selective to the liner material. On the other hand, the liner can be sufficiently thick to prevent the isotropic etch from etching the sidewalls above the etch front, thereby preventing widening of the feature other than at the etch front. While method 300 comprises applying no bias to accelerate ions to the etch front during the isotropic etch, in other examples, a suitably low bias to allow a radical-based etching process to predominate at the etch front can be applied.

[0067] Referring to 308, where the layer being etched is silicon or a stack of silicon / SiGe layers, the isotropic etch performed step 306 can include forming the plasma using a halogen-containing etchant (e.g. a chlorine or bromine-containing etchant), but omitting the oxygen-containing liner-forming substance. This can allow halogen-containing radicals to isotropically etch silicon or SiGe at the etch front, while not depositing additional liner. The previously deposited liner, with increasing thickness as a function of distance from the etch front, prevents etching of the sidewall of the feature where the liner is sufficiently thick. As a more specific example, chlorine is highly selective to silicon oxide, and does not etch silicon or SiGe with a preferential crystallographic direction. As such, a chlorine-based etch may be particularly well- suited for the isotropic etch step. Where the layer being etched is a stack of silicon nitride / silicon oxide layers or a stack of silicon / silicon oxide layers, the isotropic etch can be performed by introducing a fluorine-containing etchant. In such examples, acarbon-containing liner-forming substance can be omitted from the plasma used during the isotropic etch step.

[0068] Continuing, method 300 further comprises, at step 310, determining whether to repeat another directional etch step. The determination at step 310 can be based, for example, on whether the feature has been etched to a desired depth. If the feature has not yet been etched to a desired depth, then method 300 returns to 302 to perform another directional etch step. On the other hand, if the feature has been etched to a desired depth, method 300 can end with or without performing a final isotropic etch step at 306. In some examples, a cyclic directional etch / isotropic etch process can end after performing a final directional etch step, without performing an isotropic etch after the final directional etch step.

[0069] FIGS. 4A and 4B schematically show evolution over time of an example feature being etched using the process of FIG. 3. First, FIG. 4A shows a substrate 400. The substrate 400 can represent any suitable structure in which a HAR feature can be etched. Example structures include silicon films, stacks of alternating silicon / SiGe films, stacks of alternating silicon oxide / silicon nitride films, and stacks of alternating polysilicon / silicon oxide films.

[0070] In FIG. 4A, a feature 402 has been partially etched into the structure using a direction etch in which a bias directs reactive ions toward the substrate 400, thereby performing a vertical etch. A hard mask 403 used to define the shape of the feature 402 has not yet eroded. A shape of the etch front of the feature 402 is shown in solid line as etch front 404. Etch front 404 has a tapered profile. Further, a liner 405 has formed along a portion of the sidewall of the feature 402.

[0071] Continued etching could potentially lead to the fully formed etch feature 402 having a narrowing width as a function of increasing depth. Thus, to avoid forming such a tapered profile as a function of depth, after etching to the depth shown at etch front 404, an isotropic etch is performed to widen the etch front 404. The widened etch front after the isotropic etch is shown in dashed lines at 406. Next, directional etching is continued to a depth shown by etch front 408. Then, another isotropic etch is performed to widen the etch front to that shown in dashed lines at 410. Additional directional and isotropic etch steps are cyclically performed, thereby moving the etch front deeper within the substrate 400, as shown by solid lines 412, 416, and intermittently widening the etch front, as shown by dashed lines 414, 418. While theliner 405 is extended deeper along the sidewall with each directional etch step, the depiction of the liner is omitted from below etch front 404 for clarity.

[0072] FIG. 4B shows the fully etched feature 402 having a substantially uniform width throughout the entire depth of the feature 402. The hard mask 403 has eroded from the etching process. However, the liner 405 has protected the sidewall of the feature 402 from angular ions in the region beneath the hard mask 403. Further, the isotropic etch steps that widen the feature at the etch front help to prevent a tapered shape from forming. Thus, performing a cyclic directional etch / isotropic etch process according to the example of FIG. 3 can help to open a width of an etch front without affecting liner selectivity. This avoids the tradeoff between taper and bow encountered with directional etches that omit the intermittent isotropic etch steps. Further, by performing a longer duration and / or earlier isotropic etch step, a feature with a reentrant profile can be formed using the disclosed example cyclic directional etch / isotropic etch processes. Additionally, the directional etch steps and isotropic etch steps can be controlled to form other feature geometries, such as controlled tapers.

[0073] FIGS. 5A and 5B show graphs illustrating the width of etched features as a function of depth using both directional etch-only and cyclic directional etch / isotropic etch processes according to the disclosed examples. First, FIG. 5 A shows a depth versus width plot 500 for a reentrant feature etched using a directional etch process, and a depth versus width plot 502 for a reentrant feature etched using a cyclic directional etch / isotropic etch process. The directional etching processes were the same for both. The substrates comprised a stack of alternating silicon / SiGe layers. The isotropic etching processes were plasma-enhanced, and utilized chlorine gas (Ch) with utilized no applied bias. As shown, the plot 502 for the cyclic direction etch / isotropic etch process has a greater degree of reentrancy, as the width of this plot is higher deeper within the feature than less deep within the feature. This shows that a degree of reentrancy can be controlled by tuning the directional etch and isotropic etch steps of a cyclic directional etch / isotropic etch process.

[0074] Next, FIG. 5B shows a depth versus width plot 504 for a feature etched using a directional etch process, and a depth versus width plot 506 for a feature etched using a directional etch / isotropic etch process. In FIG. 5B, it can be seen that the width plot 506 is more uniform than the width of plot 504. As such, the uniformity of a feature width as a function of feature depth can be more accurately controlled using a cyclic directional etch / isotropic etch process than a directional etch process alone.

[0075] FIG. 6 schematically shows a processing tool that can be used to etch a feature in a substrate. The processing tool 600 can be configured for thermal etching, plasma-based etching, and / or other substrate processes.

[0076] The processing tool 600 comprises a processing chamber 602 including a showerhead 606. The processing tool 600 further includes a substrate support 608. During operation, a substrate 610 is positioned on the substrate support 608. The substrate support 608 comprises a substrate heater 612. In other examples, a heater can be omitted, or can be located elsewhere within processing chamber 602.

[0077] The processing tool 600 further comprises flow control hardware 614. The flow control hardware 614 connects processing chemical source(s) to the processing chamber. In the depicted example, the flow control hardware 614 connects one or more etchant source(s) 616, one or more liner-forming substance source(s) 618, and one or more inert gas source(s) 619 to the processing chamber. The flow control hardware 614 can include any suitable components. Examples include mass flow controllers, valves, and conduits. For example, the flow control hardware 614 can comprise one or more valves controllable to place a selected gas source or selected gas sources in fluid connection with showerhead 606 and / or a remote plasma chamber 620. As indicated at 622, flow control hardware 614 can introduce processing chemicals into a remote plasma chamber 620, The reactive species formed in the remote plasma chamber 620 pass through the showerhead 606 toward substrate 610. Additionally or alternatively, processing chemicals can be introduced into processing chamber 602 through showerhead 606, as indicated at 624.

[0078] The flow control hardware 614 also can comprise one or more mass flow controllers or other controllers for controlling a mass flow rate of gas.

[0079] The etchant source(s) 616 can comprise any suitable etchant gas. Examples include halogen-containing etchants, such as fluorine-containing etchants, chlorine-containing etchants, bromine-containing etchants, and / or interhalogens. More specific examples of halogen-containing etchants are listed above. In some more particular examples, etchant source(s) 616 can comprise one or more of HBr, HC1, Ch, F2, NO, XeF2, an interhalogen, HF, H2O, an alcohol, pyridine, NH3, NF3, SFe, WFe, MoFe, a fluorocarbon, or a fluorohydrocarbon.

[0080] The liner-forming substance source 618 comprises any suitable substance that can form a liner in an etching process that is selective to the etching chemistry. Where silicon or SiGe is being etched, example liner-forming substancesinclude oxygen-containing substances. Example oxygen-containing substances can include oxygen gas (O2), ozone (O3), carbon oxides (e.g. carbon dioxide (CO2) and carbon monoxide (CO)) nitrogen oxides (e.g. nitrous oxide (N2O)), water vapor (H2O), and hydrogen peroxide (H2O2). Where silicon oxide or silicon nitride is being etched, suitable liner-forming substances can include carbon-containing substances. Example carbon-containing substances can include alkanes having a general formula CnH2n+2 where n is an integer in a range of 1 to 10 (such as, methane, ethane, etc.), alkenes having a general formula CnEbn where n = 2 to 10 (such as, ethylene, propylene, etc.), and alkynes having a general formula CnH2n-2 where n = 2 to 10 (such as, acetylene, propyne, etc.), that are gas-phase under processing conditions. Other examples of carbon-containing substances that can be used to form an amorphous carbon liner can include aromatic hydrocarbons, cyclic aliphatic hydrocarbons, heterocyclic compounds, and alkyl amines and other nitrogen-containing compounds, that are gasphase under processing conditions and that include carbon-containing functional groups.

[0081] The inert gas source(s) can comprise any suitable inert gases. Inert gases can be used in a gas mixture comprising an etchant and a liner-forming substance, or in a gas mixture comprising an etchant but omitting a liner-forming substance. Example inert gases include helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and in some processes, nitrogen (N2).

[0082] The processing tool 600 further comprises an exhaust system 630. The exhaust system 630 is configured to exhaust gases from the processing chamber 602. The exhaust system 630 can comprise any suitable hardware, including one or more low vacuum pumps, one or more high vacuum pumps, and one or more valves for controlling an exhaust flow. Together, flow control hardware 614 and exhaust system 630 can be operated to achieve a selected pressure in processing chamber 602 during substrate processing. Further, exhaust system 630 can be operated to purge processing chamber 602.

[0083] The processing tool 600 includes a radio frequency (RF) inductive coil 632. Inductive coil 632 is used to form a transformer coupled plasma (TCP). The TCP can form reactive species from etchants and liner-forming substances introduced into the TCP. In other examples, remote plasma chamber 620 may be configured to generate a capacitively coupled plasma (CCP). As mentioned above, the reactive species formedin the remote plasma chamber 620 pass through the showerhead 606 toward substrate 610.

[0084] The processing tool 600 further comprises a RF power source 634 that is electrically connected to RF inductive coil 632. RF power source 634 is configured to form a remote TCP using a gas mixture. The processing tool 600 further includes a matching network 636 for impedance matching of the RF power source 634. The RF power source 634 can be configured to provide RF energy of any suitable frequency and power. Examples of suitable frequencies include frequencies in a range from 0.3 MHz to 10 GHz. Examples of suitable powers include powers within a range from 300 W to 2000 W. In some examples, radiofrequency power source 634 is configured to operate at a plurality of different frequencies and / or powers. In other examples, a microwave plasma may be used.

[0085] The processing tool further comprises a biasing power source 640 controllable to apply an electrical bias to accelerate ions formed in remote plasma chamber 620 toward substrate 610 for directional etching. The biasing power source 640 further can be controlled not to apply an electrical bias, and thereby allow radical- dominated isotropic etching to occur at substrate 610.

[0086] The processing tool 600 further comprises a controller 650 configured to control operation of the processing tool. Controller 650 is operatively coupled to substrate heater 612, flow control hardware 614, exhaust system 630, remote plasma chamber 620, RF power source 634, and biasing power source 640. Controller 650 further may be operatively coupled to any other suitable component of processing tool 600. Controller 650 is configured to control various functions of processing tool 600 to etch a substrate and form a feature. For example, controller 650 is configured to operate substrate heater 612 to heat a substrate. Controller 650 is also configured to operate flow control hardware 614 to flow an etchant gas mixture at a selected flow rate into remote plasma chamber 620 and / or showerhead 606. Controller 650 is also configured to operate flow control hardware 614 to flow a liner-forming substance at a selected flow rate into remote plasma chamber 620 and / or showerhead 606. Furthermore, controller 650 is configured to RF power source 634 to form a remote plasma for introducing reactive species into processing chamber 602. Controller 650 is also configured to operate exhaust system 630 to remove process gases and byproducts from processing chamber 602. Controller 650 is further configured to operate flow control hardware 614 and exhaust system 630 to maintain a selected pressure within processingchamber 602. Controller 650 is additionally configured to control biasing power source 640 to selectively apply a bias to accelerate ions formed in remote plasma chamber 620 toward substrate 610, and to selectively apply no bias or a reduced bias to perform a radical-dominated isotropic etch. Controller 650 is further configured to control any other functions of processing tool 600.

[0087] Controller 650 may comprise any suitable computing system. FIG. 7 schematically shows a non-limiting embodiment of a computing system 700 that can enact one or more of the methods and processes described above.

[0088] FIG. 7 schematically shows a non-limiting example of a computing system 700 that can enact one or more of the methods and processes described above. Computing system 700 is shown in simplified form. Computing system 700 may take the form of one or more personal computers, workstations, computers integrated with substrate processing tools, and / or network accessible server computers.

[0089] Computing system 700 includes a logic subsystem 702 and a storage subsystem 704. Computing system 700 may optionally include a display subsystem 706, input subsystem 708, communication subsystem 710, and / or other components not shown in FIG. 7. Controller 650 is an example of computing system 700.

[0090] Logic subsystem 702 includes one or more physical devices configured to execute instructions. For example, the logic subsystem may be configured to execute instructions that are part of one or more applications, services, programs, routines, libraries, objects, components, data structures, or other logical constructs. Such instructions may be implemented to perform a task, implement a data type, transform the state of one or more components, achieve a technical effect, or otherwise arrive at a desired result.

[0091] The logic subsystem may include one or more processors configured to execute software instructions. Additionally or alternatively, the logic subsystem may include one or more hardware or firmware logic subsystems configured to execute hardware or firmware instructions. Processors of the logic subsystem may be singlecore or multi-core, and the instructions executed thereon may be configured for sequential, parallel, and / or distributed processing. Individual components of the logic subsystem optionally may be distributed among two or more separate devices, which may be remotely located and / or configured for coordinated processing. Aspects of the logic subsystem may be virtualized and executed by remotely accessible, networked computing devices configured in a cloud-computing configuration.

[0092] Storage subsystem 704 includes one or more physical devices configured to hold instructions 712 executable by the logic subsystem to implement the methods and processes described herein. When such methods and processes are implemented, the state of storage subsystem 704 may be transformed — e.g., to hold different data.

[0093] Storage subsystem 704 may include removable and / or built-in devices. Storage subsystem 704 may include optical memory (e.g., CD, DVD, HD-DVD, Blu- Ray Disc, etc.), semiconductor memory (e.g., RAM, EPROM, EEPROM, etc.), and / or magnetic memory (e.g., hard-disk drive, floppy-disk drive, tape drive, MRAM, etc.), among others. Storage subsystem 704 may include volatile, nonvolatile, dynamic, static, read / write, read-only, random-access, sequential-access, location-addressable, file-addressable, and / or content-addressable devices.

[0094] It will be appreciated that storage subsystem 704 includes one or more physical devices. However, aspects of the instructions described herein alternatively may be propagated by a communication medium (e.g., an electromagnetic signal, an optical signal, etc.) that is not held by a physical device for a finite duration.

[0095] Aspects of logic subsystem 702 and storage subsystem 704 may be integrated together into one or more hardware-logic components. Such hardware-logic components may include field-programmable gate arrays (FPGAs), program- and application-specific integrated circuits (PASIC / ASICs), program- and applicationspecific standard products (PSSP / ASSPs), system-on-a-chip (SOC), and complex programmable logic devices (CPLDs), for example.

[0096] When included, display subsystem 706 may be used to present a visual representation of data held by storage subsystem 704. This visual representation may take the form of a graphical user interface (GUI). As the herein described methods and processes change the data held by the storage subsystem, and thus transform the state of the storage subsystem, the state of display subsystem 706 may likewise be transformed to visually represent changes in the underlying data. Display subsystem 706 may include one or more display devices utilizing virtually any type of technology. Such display devices may be combined with logic subsystem 702 and / or storage subsystem 704 in a shared enclosure, or such display devices may be peripheral display devices.

[0097] When included, input subsystem 708 may comprise or interface with one or more user-input devices such as a keyboard, mouse, or touch screen. In someexamples, the input subsystem may comprise or interface with selected natural user input (NUI) componentry. Such componentry may be integrated or peripheral, and the transduction and / or processing of input actions may be handled on- or off-board. Example NUI componentry may include a microphone for speech and / or voice recognition, and an infrared, color, stereoscopic, and / or depth camera for machine vision and / or gesture recognition.

[0098] When included, communication subsystem 710 may be configured to communicatively couple computing system 700 with one or more other computing devices. Communication subsystem 710 may include wired and / or wireless communication devices compatible with one or more different communication protocols. As non-limiting examples, the communication subsystem may be configured for communication via a wireless telephone network, or a wired or wireless local- or wide-area network. In some examples, the communication subsystem may allow computing system 700 to send and / or receive messages to and / or from other devices via a network such as the Internet.

[0099] It will be understood that the configurations and / or approaches described herein are exemplary in nature, and that these specific examples or examples are not to be considered in a limiting sense, because numerous variations are possible. The specific routines or methods described herein may represent one or more of any number of processing strategies. As such, various acts illustrated and / or described may be performed in the sequence illustrated and / or described, in other sequences, in parallel, or omitted. Likewise, the order of the above-described processes may be changed.

[0100] The subject matter of the present disclosure includes all novel and non- obvious combinations and sub-combinations of the various processes, systems and configurations, and other features, functions, acts, and / or properties disclosed herein, as well as any and all equivalents thereof.

Claims

CLAIMS:

1. A method of etching a feature in a substrate, the method comprising: performing a plurality of etching cycles, each etching cycle comprising performing a directional etch step in which a liner is formed along a sidewall of the feature using a liner-forming substance; and performing an isotropic etch step to widen the feature at an etch front of the feature.

2. The method of claim 1, wherein performing the directional etch comprises forming a plasma using a gas mixture comprising one or more of a chlorine-containing etchant or a bromine-containing etchant, and wherein the gas mixture also comprises an oxygen-containing liner-forming substance.

3. The method of claim 2, wherein the substrate comprises a stack of alternating layers of silicon and silicon-germanium.

4. The method of claim 2, wherein the isotropic etch step omits the oxygencontaining liner-forming substance.

5. The method of claim 1, wherein performing the directional etch step comprises applying a bias to accelerate formed in a plasma toward the substrate, and wherein performing the isotropic etch step comprises not applying the bias.

6. The method of claim 1, wherein the substrate comprises a stack of alternating layers of silicon nitride and silicon oxide, and wherein performing the directional etch step comprises using a fluorine-containing etchant and a carbon-containing linerforming substance.

7. The method of claim 1, wherein the substrate comprises a stack of alternating layers of silicon oxide and polycrystalline silicon, and wherein performing the directional etch step comprises using a fluorine-containing etchant and a carbon- containing liner-forming substance.

8. The method of claim 1, wherein performing the directional etch step comprises forming a transformer coupled plasma (TCP).

9. The method of claim 1, wherein the isotropic etch step is a plasma-enhanced etch step.

10. The method of claim 1, wherein the isotropic etch step is a thermal etch step.

11. The method of claim 1, wherein the feature has a substantially uniform width as a function of depth.

12. The method of claim 1, wherein the feature is a reentrant feature.

13. A method of etching a feature in a substrate comprising a stack of alternating layers of silicon and silicon / germanium, the method comprising: performing a cyclic directional etch / isotropic etch process for a plurality of cycles, each cycle comprising: performing a directional etch step using a halogen-containing etchant and an oxygen-containing liner-forming substance to increase a depth of the feature; and performing an isotropic etch step comprising a chlorine radical-based etching process that widens the feature at an etch front.

14. The method of claim 13, wherein the method is used in fabrication of a three- dimensional (3D) vertical dynamic random access (DRAM) memory fabrication process.

15. A processing tool, comprising: a processing chamber; a substrate support disposed in the processing chamber; a remote plasma chamber; a radiofrequency power source configured to form a plasma in the remote plasma chamber;a biasing power source configured to accelerate ions created in the remote plasma chamber toward the substrate; flow control hardware; and a controller configured to perform a plurality of etching cycles, at least one etching cycle comprising a directional etch step and an isotropic etch step, the controller configured to in the directional etch step, operate the radiofrequency power source, the biasing power source, and the flow control hardware to form a plasma comprising at least an etchant and a liner-forming substance to etch a feature of a substrate and deposit a liner along a sidewall of the feature, and in the isotropic etch step, operate the radiofrequency power source to form a plasma using at least an etchant that is selective to the liner to isotropically etch the substrate at an etch front of the feature.

16. The processing tool of claim 15, wherein the substrate comprises a stack of alternating layers of silicon nitride and silicon oxide, and wherein the controller is configured to, in the directional etch step, form a plasma comprising a fluorine- containing etchant and a carbon-containing liner-forming substance.

17. The processing tool of claim 15, wherein the substrate comprises a stack of alternating layers of silicon oxide and polycrystalline silicon, and wherein the controller is configured to, in the directional etch step, form a plasma comprising a fluorine- containing etchant and a carbon-containing liner-forming substance.

18. The processing tool of claim 15, wherein the substrate comprises a stack of alternating layers of silicon and silicon-germanium, and wherein the controller is configured to, in the directional etch step, form a plasma comprising an oxygencontaining liner-forming substance and one or more of a chlorine-containing etchant or a bromine-containing etchant.

19. The processing tool of claim 15, wherein the controller is configured to, in the isotropic etch step, form a plasma using chlorine.

20. The processing tool of claim 15, wherein the controller is configured to perform the plurality of etching cycles to form a high aspect ratio feature in the substrate.

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