Method for selectively etching silicon nitride relative to silicon oxide using water crystallization

Abstract

Disclosed herein are embodiments of improved processes and methods for selectively etching silicon nitride. More specifically, a cyclic two-step dry etching process is provided for selectively etching a silicon nitride layer formed on a substrate while protecting an oxide layer formed on the same substrate. In the cyclic two-step dry etching process, the substrate is sequentially subjected to: (1) exposure to a hydrogen plasma to modify the exposed surfaces of the silicon nitride and oxide layers to form a modified silicon nitride surface layer and a modified oxide surface layer; and (2) exposure to a halogen plasma to selectively etch the silicon nitride by removing the modified silicon nitride surface layer without removing the modified oxide surface layer. By forming a layer of crystallized water on the oxide layer during the surface modification step (i.e., step 1), the oxide layer is protected from etching during the removal step (i.e., step 2).

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to and the benefit of the filing date of U.S. Non-Provisional Patent Application No. 17 / 967,996, filed October 18, 2022, which is incorporated herein by reference in its entirety. [Background technology]

[0002] The present disclosure relates to substrate processing, and more particularly to methods for etching a layer of material on a semiconductor substrate.

[0003] The formation of semiconductor devices typically involves a series of manufacturing techniques for forming, patterning, and removing multiple layers of materials on a substrate. During typical semiconductor manufacturing, various materials formed on a substrate may be removed by patterned etching, chemical-mechanical polishing, and other techniques. Various techniques are known for etching layers on a substrate, including plasma-based or gas-phase etching (also called dry etching) and liquid-based etching (also called wet etching). As semiconductor device sizes shrink to the sub-nanoscale and device integration changes from two-dimensional to three-dimensional structures, more precise and selective etching techniques are needed for semiconductor device manufacturing.

[0004] Silicon nitride (SiN) is used in various semiconductor devices as a barrier layer for dopant diffusion, a gate sidewall spacer layer, a buffer layer, a liner layer, an etch stop layer, etc., due to its high insulating properties, high thermal stability, and mechanical stability. Because SiN layers are typically formed on substrates on which silicon (Si) and / or silicon oxide (SiO2) layers are disposed, selective etching of silicon nitride over silicon and / or silicon oxide is important for various microelectronic applications.

[0005] For example, the fabrication of nanosheet field-effect transistors (FETs) typically requires the removal of silicon nitride spacers with an etching process selective to the silicon channel region sealed by the silicon nitride spacers. Similarly, contact etch stop layers (CESLs) on epitaxial semiconductor material (epi) fins and / or along gate sidewalls typically require highly selective etching. For example, material loss from the epi fin is often required to be less than 3 nanometers (nm). In addition, removing CESL residue from the valleys located between epi diamond structures can pose significant challenges for certain process flows. Other semiconductor manufacturing processes may also require highly selective removal of silicon nitride layers to achieve desired results.

[0006] Various techniques have been used to etch silicon nitride, including fluorocarbon-based (e.g., C x F z based on), hydrofluorocarbon-based (e.g., C x H y F z based on fluorine), and other fluorine-based (e.g., NF x Etching chemistries based on C have been used to etch silicon nitride selectively to oxide, silicon, and other materials. x H y F z A CF4 / O2 / H2 plasma has been used to form a thicker deposit on SiN, and a CF4 / O2 / N2 plasma has been used to enhance nitrogen removal in the SiN layer. However, it is difficult to fine-tune the thickness of the deposited layer, and the oxide layer is still etched during this etching process. Furthermore, fluorocarbon (C x F z ) Etching gases can cause problems with carbon contamination and CF2 on the film surface, which can adversely affect device fabrication. x (CH x ) is often undesirable due to polymer deposition.

[0007] Other techniques have used a two-step approach to selectively etch silicon nitride: surface modification followed by removal of the modified surface layer. For example, one two-step process for selectively etching silicon nitride uses hydrogen plasma to modify the exposed SiN surface and dilute hydrofluoric acid (dHF) wet etching to remove the modified surface layer. This two-step process cannot be implemented as a cyclic process because it uses dry etching techniques for surface modification and wet etching techniques for removal of the modified surface layer. Another two-step process used to selectively etch silicon nitride in a cyclic dry etching process uses hydrogen plasma to modify the exposed SiN surface and fluorinated plasma to remove the modified surface layer. This two-step dry etching process is typically performed in a plasma processing chamber (e.g., an inductively coupled plasma (ICP) or capacitively coupled plasma (CCP) processing chamber) at relatively low pressures (e.g., 10-100 mTorr) and moderate to high temperatures (e.g., 10°C-100°C) to selectively etch SiN in a cyclic process.

[0008] Although selective etching processes for silicon nitride are known, current semiconductor manufacturing processes require further improvements in the etch selectivity of silicon nitride (SiN) relative to oxide (e.g., SiO2). Highly selective etching of silicon nitride relative to oxide is important in many semiconductor manufacturing processes. Therefore, improved techniques are needed to protect the oxide layer (and structures below such oxide layer) from over-etching when etching SiN on a substrate with both SiN and oxide layers formed thereon. Summary of the Invention [Means for solving the problem]

[0009] Disclosed herein are embodiments of improved processes and methods for selectively etching silicon nitride. In the disclosed embodiments, a cyclic two-step dry etching process is used to selectively etch a silicon nitride layer formed on a substrate while protecting an oxide layer formed on the same substrate. The cyclic two-step dry etching process sequentially subjects a substrate to (1) exposure to a hydrogen plasma to modify the exposed surfaces of the silicon nitride and oxide layers, forming a modified silicon nitride surface layer and a modified oxide surface layer, and (2) exposure to a halogen plasma to selectively etch the silicon nitride by removing the modified silicon nitride surface layer without removing the modified oxide surface layer. By forming a crystallized water layer on the oxide layer during the surface modification step (i.e., step 1), the oxide layer is protected from etching during the removal step (i.e., step 2). The crystallized water layer provides a protective layer on the oxide layer, preventing reactive species of the halogen plasma from reaching the modified oxide surface layer, thereby improving the selectivity of silicon nitride over oxide. The process gas used to generate the halogen plasma can further reduce (or eliminate) etching of the oxide layer by providing highly selective etching of the modified silicon nitride surface layer relative to the modified oxide surface layer.

[0010] Embodiments disclosed herein form a protective water layer on the oxide layer by taking advantage of the different volatilities of reaction byproducts generated during the surface modification step (i.e., step 1). For example, when a substrate is exposed to a hydrogen plasma, hydrogen ions in the hydrogen plasma (a) react with the exposed surface of the silicon nitride layer to form a first reaction byproduct (e.g., ammonia, NH3) and (b) react with the exposed surface of the oxide layer to form a second reaction byproduct (e.g., water, HO). By selecting appropriate pressure and temperature conditions, embodiments disclosed herein can vaporize the first reaction byproduct, leaving a silicon (Si) surface layer on the SiN layer, and freeze the second reaction byproduct, leaving a crystallized water layer on the oxide layer. Because the oxide layer is protected by the crystallized water layer, the halogen plasma selectively etches the SiN layer during the subsequent removal step (i.e., step 2).

[0011] According to one embodiment, provided herein is a method for selectively etching silicon nitride over oxide in accordance with the present disclosure. In some embodiments, the method may include providing a substrate having a silicon nitride layer and an oxide layer exposed on a surface of the substrate; and exposing the surface of the substrate to a hydrogen plasma to (a) modify the exposed surface of the silicon nitride layer to form a first modified layer and (b) modify the exposed surface of the oxide layer to form a second modified layer. The gas pressure and substrate temperature used to generate the hydrogen plasma form a layer of crystallized water, which combines with the oxide layer to form the second modified layer. The method may then include exposing the surface of the substrate to a halogen plasma to selectively etch the silicon nitride layer by removing the first modified layer without removing the second modified layer; and repeating the exposing of the surface of the substrate to the hydrogen plasma and the exposing of the surface of the substrate to the halogen plasma one or more times until a predetermined amount of the silicon nitride layer has been selectively etched.

[0012] A variety of gas pressures and substrate temperatures can be used in the above-described methods. For example, in some embodiments, the gas pressure ranges from 10 mTorr to 1500 Torr, and the substrate temperature ranges from 20°C to -150°C. In other embodiments, the gas pressure can range from 10 mTorr to 100 Torr, and the substrate temperature can range from 0°C to -80°C.

[0013] During the exposing of the surface of the substrate to the hydrogen plasma, the hydrogen plasma reacts with the exposed surface of the silicon nitride layer to form a first reaction by-product, which vaporizes at the gas pressure and temperature of the substrate to form a first modified layer. For example, the first reaction by-product can be ammonia (NH3), and the first modified layer can be a silicon layer.

[0014] During the exposure of the surface of the substrate to the hydrogen plasma, the hydrogen plasma also reacts with the exposed surface of the oxide layer to produce a second reaction by-product, which freezes at the gas pressure and the temperature of the substrate to form a crystallized water layer on the oxide layer, forming a second modified layer. For example, the second reaction by-product can be water (HO) and the second modified layer can be a silicon dioxide layer.

[0015] During the exposure of the substrate surface to the halogen plasma, the crystallized water layer can reduce etching of the oxide layer by preventing reactive species of the halogen plasma from reaching the second modified layer. In some embodiments, the first modified layer can be a silicon layer and the second modified layer can be a silicon dioxide layer. In such embodiments, the reactive species of the halogen plasma can selectively etch silicon over silicon dioxide, further reducing or eliminating etching of the oxide layer. In some embodiments, the method can further include generating a halogen plasma from one or more process gases having an etch selectivity of silicon to silicon dioxide of 5:1 or greater. For example, the one or more process gases used to generate the halogen plasma can include one or more of chlorine (Cl), hydrogen bromide (HBr), difluorine (F), xenon difluoride (XeF), tetrafluoromethane (CF), nitrogen trifluoride (NF), and sulfur hexafluoride (SF).

[0016] According to another embodiment, another method for selectively etching silicon nitride over oxide in accordance with the present disclosure is provided herein. In some embodiments, the method may include providing a substrate having a silicon nitride layer and an oxide layer exposed on a surface of the substrate; generating a hydrogen plasma at a gas pressure of less than 1500 Torr and a substrate temperature of 0° C. or less; and exposing the surface of the substrate to the hydrogen plasma. Upon exposure to the hydrogen plasma, the hydrogen plasma (a) reacts with the exposed surface of the silicon nitride layer to form a first reaction by-product, which vaporizes at the gas pressure and substrate temperature to form a first modified layer on the silicon nitride layer, and (b) reacts with the exposed surface of the oxide layer to form a second reaction by-product, which freezes at the gas pressure and substrate temperature to form a crystallized water layer, forming a second modified layer on the oxide layer. The method may further include exposing the surface of the substrate to a halogen plasma to selectively etch the silicon nitride layer by removing the first modified layer without removing the second modified layer. In some embodiments, the method may further include repeating the exposing the surface of the substrate to a hydrogen plasma and the exposing the surface of the substrate to a halogen plasma multiple times to selectively etch a predetermined amount of the silicon nitride layer.

[0017] As mentioned above, a variety of gas pressures and substrate temperatures can be used in the methods described above. In some embodiments, the gas pressure can range from 10 mTorr to 1500 Torr, and the substrate temperature can range from 0°C to -80°C.

[0018] During the exposure of the substrate surface to the halogen plasma, the crystallized water layer can reduce etching of the oxide layer by preventing reactive species of the halogen plasma from reaching the second modified layer. In some embodiments, the first modified layer can be a silicon layer and the second modified layer can be a silicon dioxide layer. In such embodiments, the reactive species of the halogen plasma can selectively etch silicon over silicon dioxide, further reducing or eliminating etching of the oxide layer. In some embodiments, the halogen plasma can be generated from one or more process gases having an etch selectivity of silicon to silicon dioxide of 5:1 or greater.

[0019] A variety of process gases can be used to generate the hydrogen and halogen plasmas described herein. For example, hydrogen plasma can be generated from one or more process gases including, but not limited to, hydrogen (H), hydrogen bromide (HBr), methane (CH), and hydrogen sulfide (HS). Halogen plasma can be generated from one or more process gases including, but not limited to, chlorine (Cl), hydrogen bromide (HBr), difluorine (F), xenon difluoride (XeF), tetrafluoromethane (CF), nitrogen trifluoride (NF), and sulfur hexafluoride (SF).

[0020] In some embodiments, the first reaction by-product can be ammonia (NH), the first modified layer can be a silicon layer, the second reaction by-product can be water (HO), and the second modified layer can be a silicon dioxide layer. When the surface of the substrate is exposed to the hydrogen plasma, the gas pressure and substrate temperature used to generate the hydrogen plasma can (a) vaporize ammonia (NH) from the exposed surface of the silicon nitride layer to form a silicon layer, and (b) freeze water (HO) on the exposed surface of the oxide layer to form a crystallized water layer that combines with the exposed surface of the oxide layer to form a silicon dioxide layer. As described above, the gas pressure and substrate temperature used to generate the hydrogen plasma can be selected from a pressure-temperature window including a pressure range of 10 mTorr to 1500 Torr and a temperature range of 0°C to -80°C.

[0021] The present invention and its advantages will be more fully understood by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals indicate like features, and in which it should be noted, however, that the accompanying drawings illustrate only exemplary embodiments of the disclosed concepts and are not intended to limit the scope of the invention, as the disclosed concepts may encompass other embodiments that are equally effective. [Brief explanation of the drawings]

[0022] [Figure 1] FIG. 1 is a process flow diagram of an exemplary embodiment of a cyclic two-step dry etching process in which plasma etching steps using a hydrogen plasma (step 1) followed by a halogen plasma (step 2) are applied sequentially to selectively etch silicon nitride relative to oxide. [Figure 2] FIG. 1 is a process flow diagram of an exemplary embodiment of a cyclic two-step dry etching process that includes: (1) modifying exposed surfaces of silicon nitride and oxide layers using a hydrogen plasma to form a modified silicon nitride surface layer and a modified oxide surface layer; and (2) selectively etching the silicon nitride layer by using a halogen plasma to remove the modified silicon nitride surface layer without removing the modified oxide surface layer. [Figure 3] 1 is a phase diagram of ammonia (NH3) and water (HO) showing the pressure-temperature window that can be used to select appropriate pressure and temperature conditions for the cyclic two-step dry etching process described herein. [Figure 4] FIG. 1 is a flow chart diagram illustrating one embodiment of a method for selectively etching silicon nitride over oxide utilizing the techniques described herein. [Figure 5] FIG. 1 is a flow chart diagram illustrating another embodiment of a method for selectively etching silicon nitride over oxide utilizing the techniques described herein. [Figure 6] 1 is a block diagram illustrating one embodiment of a plasma processing apparatus that can be used as an etch chamber in embodiments described herein. DETAILED DESCRIPTION OF THE INVENTION

[0023] The present disclosure provides various embodiments of improved processes and methods for selectively etching silicon nitride over oxide. In the disclosed embodiments, a cyclic two-step dry etching process is used to selectively etch a silicon nitride layer formed on a substrate while protecting an oxide layer formed on the same substrate. In the cyclic two-step dry etching process disclosed herein, the substrate is sequentially subjected to the following steps: (1) exposing the substrate to a hydrogen plasma to modify the exposed surfaces of the silicon nitride and oxide layers to form a modified silicon nitride surface layer and a modified oxide surface layer; and (2) exposing the substrate to a halogen plasma to selectively etch the silicon nitride by removing the modified silicon nitride surface layer without removing the modified oxide surface layer. By forming a layer of crystallized water on the oxide layer during the surface modification step (i.e., step 1), the oxide layer is protected from etching during the removal step (i.e., step 2). The crystallized water layer provides a protective layer on the oxide layer, preventing reactive species of the halogen plasma from reaching the modified oxide surface layer, thereby improving the selectivity of silicon nitride over oxide. The process gas used to generate the halogen plasma can further reduce (or eliminate) etching of the oxide layer by providing highly selective etching of the modified silicon nitride surface layer over the modified oxide surface layer.

[0024] As described in more detail below, embodiments disclosed herein form a protective water layer on an oxide layer by taking advantage of the differing volatilities of reaction byproducts generated during the surface modification step (i.e., step 1). For example, when a substrate is exposed to a hydrogen plasma during the surface modification step, hydrogen ions in the hydrogen plasma (a) react with the exposed SiN surface to form a first reaction byproduct (e.g., ammonia, NH3) and (b) react with the exposed oxide surface to form a second reaction byproduct (e.g., water, HO). During the surface modification step, pressure and temperature conditions are carefully selected to vaporize the NH3, leaving a silicon (Si) surface layer on the SiN layer, and freeze the HO, leaving a crystallized water layer on the oxide layer. The crystallized water layer protects the oxide layer during the subsequent removal step (i.e., step 2), thereby providing a two-step dry etching process for etching SiN over oxide with high etch selectivity. Because the dry etching process is utilized for both surface modification and removal, cyclical repetition of the two-step process described herein can selectively etch a predetermined amount of SiN without significantly etching the oxide.

[0025] FIG. 1 is a process flow diagram of an exemplary embodiment of a cyclic two-step dry etching process 100 that can be used to selectively etch silicon nitride relative to oxide in accordance with the techniques described herein. The cyclic two-step dry etching process 100 shown in FIG. 1 can be used to selectively etch a silicon nitride layer formed on a substrate while protecting an oxide layer formed on the same substrate. The dry etching process shown in FIG. 1 can be performed on a variety of substrates, which can be placed in a variety of plasma processing systems. Examples of plasma processing systems that can be used to perform the dry etching process shown in FIG. 1 include, but are not limited to, a capacitively coupled plasma (CCP) processing system, an inductively coupled plasma (ICP) processing system, a microwave plasma processing system, a radial line slot antenna (RLSA) processing system, and a plasma processing system. TM) microwave plasma processing systems, electron cyclotron resonance (ECR) plasma processing systems, or other types of processing systems or combinations of systems.

[0026] The dry etching process 100 shown in FIG. 1 is generally a cyclic two-step process including a surface modification step (step 1) followed by a removal step (step 2). During the surface modification step (step 1), the dry etching process 100 generates a hydrogen plasma in a plasma processing chamber at a gas pressure of less than 1500 Torr and a substrate temperature of less than 0° C. (Block 110) and exposes a substrate having a silicon nitride (SiN) layer and an oxide (Ox) layer (e.g., SiO2) formed thereon to the hydrogen plasma to form a first modification layer on the SiN layer and a second modification layer on the oxide layer (Block 120). When the substrate is exposed to the hydrogen plasma (Block 120), hydrogen ions in the hydrogen plasma (a) react with the exposed SiN surface to remove nitrogen (N) atoms from the exposed SiN surface and (b) react with the exposed oxide surface to remove oxygen (O) atoms from the exposed oxide surface. This reaction modifies the exposed SiN and exposed oxide surfaces, leaving a first modified layer (e.g., a silicon layer) on the unmodified SiN layer and a second modified layer (e.g., an O-deficient silicon dioxide layer) on the unmodified oxide layer.

[0027] During the removal step (step 2), the dry etching process 100 generates a halogen plasma in the plasma processing chamber (block 130) and exposes the substrate to the halogen plasma (block 140) to selectively etch the silicon nitride layer by removing the first modified layer without removing the second modified layer. When the substrate is exposed to the halogen plasma (block 140), the first modified layer is etched at a much higher etch rate than the second modified layer, selectively etching the modified portion of the silicon nitride layer. After the modified portion of the silicon nitride layer has been removed, the surface modification step (step 1) and the removal step (step 2) can be repeated, as indicated by arrow 150, to remove a desired amount of silicon nitride from the substrate.

[0028] 2 illustrates the surface modification step (Step 1) and the removal step (Step 2) in more detail. When a substrate including a SiN layer 200 and an oxide (Ox) layer 220 (e.g., SiO2) is exposed to a hydrogen plasma 250 during the surface modification step (Step 1), hydrogen ions in the hydrogen plasma 250 react with nitrogen (N) atoms on the exposed surface of the SiN layer 200 to form a first reaction by-product (e.g., ammonia, NH3), which vaporizes at the gas pressure and substrate temperature in the plasma processing chamber to form a silicon (Si) layer 210 (i.e., the first modified layer) on the underlying unmodified portion of the SiN layer 200. The hydrogen ions in the hydrogen plasma 250 also react with oxygen (O) atoms on the exposed surface of the oxide (Ox) layer 220 to form a second reaction by-product (e.g., water, HO), which condenses and freezes at the gas pressure and substrate temperature in the plasma processing chamber to form a crystallized water layer 230 on the oxide layer 220. The crystallized water layer 230 forms an O-deficient silicon dioxide (SiO ) layer on the underlying unmodified oxide layer 220 due to the formation of Si—O bonds, rather than a Si layer. 2-x ) layer 240 (i.e., second modified layer).

[0029] When the substrate is exposed to halogen plasma 260 during the removal step (step 2), the crystallized water layer 230 is converted by reactive species of the halogen plasma 260 into O-defective silicon dioxide (SiO 2-x ) layer 240 (i.e., the second modified layer) and the underlying unmodified oxide layer 220, thereby protecting the oxide layer 220 from etching. In addition, the process gas used to generate the halogen plasma 260 is O-defective silicon dioxide (SiO 2-xEtching of the oxide layer 220 can be further reduced (or eliminated) by increasing the etch selectivity of the Si layer 210 (i.e., the first modified layer) relative to the SiO2 layer 240 (i.e., the second modified layer). For example, the process gas used to generate the halogen plasma 260 can have a Si to SiO2 etch selectivity of 5:1, or more preferably 10:1 or greater. While a variety of halogen-containing gases can be used in the removal step (step 2), examples of process gases that provide high Si to SiO2 etch selectivity include, but are not limited to, hydrogen bromide (HBr), chlorine (Cl2), difluorine (F2), xenon difluoride (XeF2), and other halogen-containing process gases. This allows for the removal of O-defective silicon dioxide (SiO2) during the removal step (step 2). 2-x ) layer 240.

[0030] After removing the Si layer 210 and selectively etching the SiN layer 200, the surface modification step (step 1) and the removal step (step 2) can be repeated one or more times (as needed) to remove a desired amount of SiN from the substrate, as indicated by arrow 270. In some embodiments, a predetermined amount of the SiN layer 200 can be removed by repeating the surface modification step (step 1) and the removal step (step 2) N times (where N ranges from 1 to 100), as shown in FIG.

[0031] Dry etching process 100 provides an example of an improved process that can be used to selectively etch silicon nitride over oxide. As described above with respect to FIGS. 1-2, dry etching process 100 utilizes sequential surface modification and removal steps to selectively etch a silicon nitride layer on a substrate without etching an oxide layer on the same substrate. Note that the silicon nitride layer can be implemented as silicon nitride (SiN) or as a silicon nitride material containing SiN and one or more other elements. In one example, the silicon nitride layer can be a silicon carbonitride / boron nitride (SiBCN) spacer. Other silicon nitride materials can also be selectively etched using the dry etching process 100 shown in FIGS. 1 and 2.

[0032] In addition to providing high selectivity of silicon nitride over silicon oxide, the dry etching process 100 can be repeated multiple times to etch any desired amount of silicon nitride. Note that the surface modification step (Step 1) tends to have a practical limit to the depth of modified nitride that can be produced by a single exposure of the silicon nitride layer to a hydrogen plasma. Therefore, surface modification and removal steps are typically used sequentially to remove a predetermined or desired amount of the silicon nitride layer from the surface of the substrate.

[0033] 1 and 2 may include additional process steps before and / or after the surface modification step (step 1) and / or the removal step (step 2). For example, a purge step may be performed between the surface modification step (step 1) and / or the removal step (step 2) to remove process gases from the processing space of the plasma processing chamber. Other variations may also be implemented while still utilizing the techniques described herein.

[0034] A variety of plasma processing systems, parameters, and process conditions can be used to selectively etch a silicon nitride layer over an oxide layer formed on a substrate according to the techniques described herein. For example, a plasma processing apparatus (such as a CCP or ICP processing system) capable of injecting and igniting plasma process gases into a processing chamber can be used. The plasma processing apparatus can also be capable of applying a selected gas pressure (P) within the processing chamber, applying a selected temperature (T) to the substrate, applying selected radio frequency (RF) energy to the processing chamber, and controlling the duration of the plasma processing step.

[0035] A variety of process gases can be utilized to generate the hydrogen plasma used in step 1 and the halogen plasma used in step 2 under a variety of processing conditions. For example, the hydrogen plasma can be generated by injecting one or more hydrogen-containing gases, such as hydrogen (H), hydrogen bromide (HBr), methane (CH), or hydrogen sulfide (HS), into the processing chamber. When generating the hydrogen plasma according to the techniques described herein, the processing chamber is maintained at a gas pressure (e.g., less than 1500 Torr) and substrate temperature (e.g., less than 0° C.) sufficient to vaporize the first reaction by-product (e.g., NH) and freeze or crystallize the second reaction by-product (e.g., HO) during the surface modification step (step 1). As mentioned above, a halogen plasma can be generated by injecting one or more halogen-containing gases, such as hydrogen bromide (HBr), chlorine (Cl), difluorine (F), xenon difluoride (XeF), tetrafluoromethane (CF), nitrogen trifluoride (NF), or sulfur hexafluoride (SF), into the processing chamber. When generating a halogen plasma, the gas pressure and substrate temperature in the processing chamber are preferably set higher and lower than those used to generate the hydrogen plasma process in order to preserve the protective crystallized water layer 230 intact.

[0036] In one exemplary embodiment, the following parameters set forth in the table below may be used in the dry etching process 100 shown in Figures 1-2 to selectively etch a silicon nitride layer over an oxide layer in accordance with the techniques described herein. In the table below, gas pressure is expressed in Torr, substrate temperature is expressed in degrees Celsius (°C), radio frequency (RF) energy is expressed in watts (W), and duration is expressed in seconds (sec). While exemplary process conditions are provided below for illustrative purposes, other variations and process parameters may be used while utilizing the techniques described herein.

[0037] [Table 1]

[0038] As shown in the table above, the hydrogen plasma used in step 1 can be generated in one embodiment by supplying hydrogen (H) gas and argon (Ar) gas into a process chamber maintained at a gas pressure range of 0.01 to 1500 Torr and a substrate temperature range of -150°C to 20°C. In some embodiments, as further described herein, the gas pressure can be in the range of 0.01 to 100 Torr, and the substrate temperature can be in the range of -80°C to 0°C. While the process gas is being supplied to the process chamber, RF energy of 10 W to 1000 W can be supplied to one or more electrodes to ignite the process gas and generate hydrogen plasma in the process chamber. The surface modification step (step 1) shown in FIGS. 1 and 2 can be performed by exposing a substrate placed in the process chamber to the hydrogen plasma for approximately 5 to 300 seconds. While Table 1 shows an example of a hydrogen-containing gas (e.g., H), other hydrogen-containing gases and / or inert gases can also be supplied to the process chamber to generate the hydrogen plasma used in step 1.

[0039] As shown in the table above, in one embodiment, the halogen plasma used in step 2 can be generated by supplying hydrogen bromide (HBr) gas into a process chamber maintained at a gas pressure range of 0.01 to 1500 Torr and a substrate temperature range of -150°C to 20°C. While similar gas pressure and temperature ranges are provided for generating hydrogen and halogen plasmas, higher gas pressures and lower substrate temperatures may be selected from the above ranges for generating halogen. While the process gas is supplied to the process chamber, RF energy of 10 W to 1000 W can be supplied to one or more electrodes to ignite the process gas and generate halogen plasma within the process chamber. A substrate placed in the process chamber can be exposed to the halogen plasma for approximately 5 to 180 seconds to perform the removal step (step 2) shown in Figures 1 and 2. While Table 1 shows an example of a halogen-containing gas (e.g., HBr), other halogen-containing gases and / or inert gases can also be supplied to the process chamber to generate the halogen plasma used in step 2.

[0040] The dry etching process 100 shown in Figures 1 and 2 takes advantage of the different volatilities of the reaction by-products (e.g., NH3 and HO) produced during the surface modification step (step 1) to improve the dry etching selectivity of silicon nitride over oxide and prevent etching of the oxide layer. Figure 3 shows a phase diagram 300 for ammonia (NH3) and water (HO) at various gas pressures (Torr) and substrate temperatures (°C). As shown in phase diagram 300, ammonia (NH3) vaporizes into a gas at gas pressures and substrate temperatures below phase curve 310 and condenses into a liquid at gas pressures and substrate temperatures above phase curve 310. Similarly, water (HO) vaporizes into a gas at gas pressures and substrate temperatures below phase curve 320, condenses into a liquid at gas pressures above phase curve 320 and substrate temperatures above 0°C, and freezes (or crystallizes) into a solid at gas pressures above phase curve 320 and substrate temperatures below 0°C.

[0041] 1 and 2 utilizes the different volatilities of ammonia (NH) and water (HO) to ensure that NH evaporates from the exposed SiN surface during the surface modification step (Step 1) and HO crystallizes on the exposed oxide surface. The evaporation of NH forms a modified surface layer (e.g., a Si layer) on the exposed SiN surface, which is easily removed by halogen plasma during the removal step (Step 2). However, the crystallized water layer formed on the exposed oxide surface during the surface modification step (Step 1) protects the underlying oxide layer from etching and improves etch selectivity during the subsequent removal step (Step 2).

[0042] 4 and 5 illustrate exemplary method embodiments for selectively etching a silicon nitride layer over an oxide layer formed on a substrate using the techniques described herein. It will be recognized that the embodiments illustrated in FIGS. 4 and 5 are exemplary only, and that additional methods may employ the techniques described herein. Furthermore, the process steps described are not intended to be exclusive, and additional process steps may be added to the methods illustrated in FIGS. 4 and 5. Furthermore, the order of steps is not limited to the order shown, as different orders may occur and / or various steps may be combined or performed simultaneously.

[0043] Figure 4 illustrates one embodiment of a method 400 for selectively etching a silicon nitride layer over an oxide layer formed on a substrate using the techniques described herein. As shown in Figure 4, method 400 may include providing a substrate having a silicon nitride layer and an oxide layer exposed on a surface of the substrate (step 410), and exposing the surface of the substrate to a hydrogen plasma (step 420) to (a) modify the exposed surface of the silicon nitride layer to form a first modified layer, and (b) modify the exposed surface of the oxide layer to form a second modified layer, wherein the gas pressure used to generate the hydrogen plasma and the temperature of the substrate form a layer of crystallized water that combines with the oxide layer to form the second modified layer. The method 400 may further include exposing the surface of the substrate to a halogen plasma to selectively etch the silicon nitride layer by removing the first modified layer without removing the second modified layer (step 430), and repeating the exposing the surface of the substrate to a hydrogen plasma and the exposing the surface of the substrate to a halogen plasma one or more times until a predetermined amount of the silicon nitride layer has been selectively etched (step 440).

[0044] FIG. 5 illustrates another embodiment of a method 500 for selectively etching a silicon nitride layer over an oxide layer formed on a substrate using the techniques described herein. As shown in FIG. 5, method 500 can begin by providing a substrate having a silicon nitride layer and an oxide layer exposed on the surface of the substrate (step 510). Next, method 500 can include generating a hydrogen plasma at a gas pressure less than 1500 Torr and a substrate temperature of 0° C. or less (step 520) and exposing the surface of the substrate to the hydrogen plasma (step 530). In step 530, the hydrogen plasma (a) reacts with the exposed surface of the silicon nitride layer to form a first reaction by-product, which vaporizes at the gas pressure and substrate temperature to form a first modified layer on the silicon nitride layer, and (b) reacts with the exposed surface of the oxide layer to form a second reaction by-product, which freezes at the gas pressure and substrate temperature to form a crystallized water layer, forming a second modified layer on the oxide layer. After exposing the surface of the substrate to the hydrogen plasma (step 530), the method 500 may further include exposing the surface of the substrate to a halogen plasma (step 540) to selectively etch the silicon nitride layer by removing the first modified layer without removing the second modified layer. In some embodiments, the method 500 may further include repeating the exposing the surface of the substrate to the hydrogen plasma (step 530) and the exposing the surface of the substrate to the halogen plasma (step 540) multiple times until a predetermined amount of the silicon nitride layer has been selectively etched.

[0045] 4 and 5 provide various examples of improved methods that can be used to selectively etch silicon nitride over oxide. Methods 400 and 500 improve the selectivity of silicon nitride over oxide by forming a crystallized water layer on the oxide layer during the surface modification steps (i.e., steps 420 and 530) used to form the first and second modified layers. When the surface of the substrate is subsequently exposed to a halogen plasma during the removal step (i.e., steps 430 and 540), the crystallized water layer prevents reactive species of the halogen plasma from reaching the second modified layer, reducing etching of the oxide layer.

[0046] Similar to the dry etching process 100 shown in Figures 1 and 2, the methods 400, 500 shown in Figures 4 and 5 improve the dry etching selectivity of silicon nitride over oxide and prevent etching of the oxide layer by taking advantage of the different volatilities of the reaction byproducts generated during the surface modification steps (i.e., steps 420 and 530). For example, when the surface of a substrate is exposed to a hydrogen plasma (steps 420 and 530), the hydrogen plasma reacts with the exposed surface of the silicon nitride layer to form a first reaction byproduct (e.g., ammonia, NH), which vaporizes at the gas pressure and substrate temperature to form a first modified layer (e.g., a silicon layer) on the silicon nitride layer. The hydrogen plasma also reacts with the exposed surface of the oxide layer to form a second reaction byproduct (e.g., water, HO), which freezes at the gas pressure and substrate temperature to form a crystallized water layer on the oxide layer and form a second modified layer (e.g., a silicon dioxide layer) on the oxide layer.

[0047] The gas pressure and substrate temperature used to generate the hydrogen plasma are selected from a pressure-temperature window that includes (a) vaporizing ammonia (NH) from the exposed surface of the silicon nitride layer to form a silicon layer, and (b) freezing water (HO) on the exposed surface of the oxide layer to form a crystallized water layer that combines with the exposed surface of the oxide layer to form a silicon dioxide layer. In some embodiments, the gas pressure and substrate temperature used to generate the hydrogen plasma can be selected from a pressure-temperature window including a pressure range of 10 mTorr to 1500 Torr and a temperature range of 20°C to -150°C. In other embodiments, the gas pressure and substrate temperature used to generate the hydrogen plasma can be selected from a pressure-temperature window including a pressure range of 10 mTorr to 100 Torr and a temperature range of 0°C to -80°C.

[0048] In some embodiments, methods 400 and 500 can further reduce (or eliminate) etching of the oxide layer by selecting appropriate process gases used to selectively etch the silicon nitride layer in steps 430 and 540. As described above, the first modified layer can be a silicon layer, and the second modified layer can be a silicon dioxide layer. In such embodiments, the reactive species of the halogen plasma can be selected to selectively etch silicon over silicon dioxide, further reducing or eliminating etching of the oxide layer. Examples of process gases that can be used to selectively etch silicon over silicon dioxide include, but are not limited to, halogen-containing gases such as chlorine (Cl), hydrogen bromide (HBr), difluorine (F), and xenon difluoride (XeF). Other halogen-containing gases with an etch selectivity of silicon to silicon dioxide of 5:1 or greater can also be used to generate the halogen plasma. For example, tetrafluoromethane (CF), nitrogen trifluoride (NF), or sulfur hexafluoride (SF) can also be used to generate the halogen plasma.

[0049] Improved processes and methods for selectively etching a silicon nitride layer over an oxide layer formed on a substrate have been described above. It should be noted that one or more deposition processes known to those skilled in the art can be used to form the silicon nitride and oxide layers described herein. For example, one or more depositions can be performed using chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), physical vapor deposition (PVD), atomic layer deposition (ALD), and / or other deposition processes using any wide range of depositions. Lithography processing of the PR layer can be performed using optical lithography, extreme ultraviolet (EUV) lithography, and / or other lithography processes. The etching process utilized to selectively etch silicon nitride over oxide can be performed using a plasma etching process, a discharge etching process, and / or other desired etching process. For example, the plasma etching process can be performed using a hydrogen-containing gas and a halogen-containing gas in combination with one or more diluent gases (e.g., argon, nitrogen, etc.), although other techniques may also be utilized. Additionally, operating variables of the process step can be controlled to achieve desired etching parameters. Operating variables may include, for example, chamber temperature, chamber pressure, gas flow rate, frequency and / or power applied to the electrode assembly in generating the plasma, and / or other operating variables for the processing step. Variations can also be implemented while still utilizing the techniques described herein.

[0050] As mentioned above, the improved processes and methods can be implemented using a variety of plasma processing systems. Referring to Figure 6, one embodiment of an exemplary plasma processing system 600 is described. However, it should be noted that the techniques described herein can be utilized in a wide variety of plasma processing systems, and that plasma processing system 600 is merely one exemplary embodiment.

[0051] FIG. 6 illustrates a block diagram of an example plasma processing system 600 that can be used as an etch chamber for embodiments described herein. More specifically, FIG. 6 illustrates a schematic cross-sectional view of a capacitively coupled plasma (CCP) processing system 600 that includes a processing space (PS) or processing chamber for receiving and processing a semiconductor substrate. While FIG. 6 illustrates a CCP processing system 600, alternative plasma processing systems, such as, but not limited to, an inductively coupled plasma (ICP) processing system or a microwave plasma processing system, can also be utilized. CCP processing systems may be particularly well-suited for implementing the techniques described herein because the electrode spacing of a CCP processing system allows for beneficial control of gases to localized regions of the plasma space, thereby enabling localized plasma processing on the substrate.

[0052] The plasma processing system 600 can be used for multiple operations, including ashing, etching, deposition, cleaning, plasma polymerization, plasma-enhanced chemical vapor deposition (PECVD), atomic layer deposition (ALD), and the like. The structure of the CCP processing system 600 is well known, and the specific structure presented herein is merely exemplary. For example, plasma processing can be performed in a processing chamber 601, which can be a vacuum chamber made of a metal such as aluminum or stainless steel. The processing chamber 601 defines a processing vessel that provides a process space (PS) for plasma generation. The interior walls of the processing vessel can be coated with alumina, yttria, or other protective agents. The processing vessel can be cylindrical in shape or have other geometric configurations.

[0053] In the lower central region of the processing chamber 601, a susceptor 612 (which may be disk-shaped) can function as, for example, a mounting table on which a substrate 602 (such as a semiconductor wafer) to be processed can be mounted. The substrate 602 can be moved into the processing chamber 601 via a loading / unloading port and a gate valve. The susceptor 612 forms part of a lower electrode assembly 620 as an example of a second electrode that functions as a mounting table for mounting the substrate 602 thereon. The susceptor 612 can be formed, for example, from an aluminum alloy. The susceptor 612 is provided with an electrostatic chuck (as part of the lower electrode assembly) for holding the substrate 602. The electrostatic chuck is provided with an electrode 635. The electrode 635 is electrically connected to a direct current (DC) power supply (not shown). The electrostatic chuck attracts the substrate 602 thereto via an electrostatic force generated when a DC voltage from the DC power supply is applied to the electrode 635. The susceptor 612 can be electrically connected to a radio frequency power supply via a matching unit. In other embodiments and processing chambers, two or more power sources can be used and connected to electrode 635 and / or other electrodes in the processing chamber. This RF power source (second power source) can output a RF voltage, for example, in the range of 2 MHz (megahertz) to 20 MHz. By applying the RF bias power, ions in the plasma generated in processing chamber 601 are attracted to substrate 602. A focus ring assembly 638 is provided on the upper surface of susceptor 612 to surround the electrostatic chuck.

[0054] The exhaust path 633 can be formed through one or more exhaust ports (not shown) connected to a gas exhaust unit. The gas exhaust unit can include a vacuum pump, such as a turbomolecular pump, configured to evacuate the plasma processing space within the processing chamber 601 to a desired vacuum state. The gas exhaust unit evacuates the processing chamber 601, thereby reducing its internal pressure to the desired vacuum level.

[0055] The upper electrode assembly 670 is an example of a first electrode and is disposed vertically above the lower electrode assembly 620 so as to face parallel to the lower electrode assembly 620. A plasma generation space, i.e., a process space (PS), is defined between the lower electrode assembly 620 and the upper electrode assembly 670. The upper electrode assembly 670 includes an inner upper electrode 671 having a disk shape and an outer upper electrode, which may be annular, surrounding the inner upper electrode 671. The inner upper electrode 671 also functions as a process gas inlet for injecting a specific amount of process gas into the process space (PS) above the substrate 602 attached to the lower electrode assembly 620. In this way, the upper electrode assembly 670 forms a showerhead. More specifically, the inner upper electrode 671 includes a gas injection opening 682.

[0056] The upper electrode assembly 670 may include one or more buffer chambers 689A, 689B, and 689C. The buffer chambers may be used to diffuse process gases and define a disk-shaped space. Process gases from a process gas supply system 680 are supplied to the upper electrode assembly 670. The process gas supply system 680 may be configured to supply process gases to perform specific processes, such as film formation or etching, on the substrate 602. The process gas supply system 680 is connected to gas supply lines 681A, 681B, and 681C, which form a process gas supply path. The gas supply lines are connected to the buffer chambers of the inner upper electrode 671. The process gases can then travel from the buffer chambers to gas injection openings 682 on their undersides. The flow rates of the process gases introduced into the buffer chambers 689A-C can be adjusted, for example, using mass flow controllers. The introduced process gases are then exhausted to the processing space (PS) through the gas injection openings 682 in the electrode plate (showerhead electrode). The inner upper electrode 671 functions in part to provide a showerhead electrode assembly.

[0057] As shown in FIG. 6 , three buffer chambers 689A, 689B, and 689C are provided, corresponding to edge buffer chamber 689A, middle buffer chamber 689B, and central buffer chamber 689C. Similarly, gas supply lines 681A, 681B, and 681C can be configured as edge gas supply line 681A, middle gas supply line 681B, and central gas supply line 681C. The buffer chambers are provided to correspond to different localized regions of the substrate, in this case, edge, middle, and center. As described further below, these regions can correspond to specific process plasma processing conditions for the localized regions of the substrate 602. It will be appreciated that the use of three localized regions is merely exemplary. Thus, the plasma processing apparatus can be configured to provide localized plasma processing conditions to any number of regions of the substrate. Furthermore, any of a variety of configurations can be utilized, and it should again be noted that the techniques described herein are not limited to how process gas supply system 680 is configured to divide the gas flow to the various buffer chambers.

[0058] The upper electrode assembly 670 is electrically connected to a radio frequency power supply (not shown) (first radio frequency power supply) via a power supply 665 and a matching unit 668. The radio frequency power supply can output a radio frequency voltage having a frequency of 40 MHz (megahertz) or higher (e.g., 60 MHz), or can output a very high frequency (VHF) voltage having a frequency of 30-300 MHz. This power supply is sometimes referred to as the main power supply in comparison to the bias power supply. Note that in certain embodiments, there is no power supply for the upper electrode, and two power supplies are connected to the lower electrode. Other variations are possible.

[0059] The components of the plasma processing apparatus may be connected to and controlled by a control unit, which may be connected to corresponding memory storage units and a user interface (neither shown). Various plasma processing operations may be performed via the user interface, and various plasma processing recipes and operations may be stored in the storage unit. Thus, a given substrate may be processed using various micromachining techniques within the plasma processing chamber. During operation, the plasma processing apparatus generates a plasma within a processing space (PS) using an upper electrode and a lower electrode. This generated plasma may then be used to process a target substrate (such as substrate 602 or any material to be processed) in various types of processes, including plasma etching, chemical vapor deposition, and processing of semiconductor materials, glass materials, and large panels such as thin-film solar cells, other solar cells, and organic / inorganic plates for flat panel displays.

[0060] It should be noted that the phrase "one embodiment" or "one embodiment" used throughout this specification means that a particular feature, structure, material, or characteristic described with respect to that embodiment is included in at least one embodiment of the present invention, but does not imply that it is present in all embodiments. Thus, the appearances of the phrase "in one embodiment" or "in one embodiment" in various places throughout this specification do not necessarily refer to the same embodiment of the present invention. Furthermore, particular features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments. In other embodiments, various additional layers and / or structures may be included and / or described features may be omitted.

[0061] The term "substrate" as used herein means and includes a substrate or substructure upon which a material is formed. It will be understood that a substrate may include a single material, multiple layers of different materials, a single layer or multiple layers having regions of different materials or structures therein, etc. These materials may include semiconductors, insulators, conductors, or combinations thereof. For example, a substrate may be a semiconductor substrate, a base semiconductor layer on a support structure, metal electrodes, or a semiconductor substrate having one or more layers, structures, or regions formed thereon. A substrate may be a conventional silicon substrate or other bulk substrate including a layer of a semiconducting material. As used herein, the term "bulk substrate" means and includes not only silicon wafers, but also silicon-on-insulator ("SOI") substrates, such as silicon-on-sapphire ("SOS") substrates and silicon-on-glass ("SOG") substrates, epitaxial layers of silicon on a base semiconductor substrate, and other semiconductor or optoelectronic materials, such as silicon germanium, germanium, gallium arsenide, gallium nitride, and indium phosphide. The substrate may be doped or undoped.

[0062] Systems and methods for processing substrates have been described in various embodiments. Those skilled in the art will understand that the various embodiments may be practiced without one or more of the specific details, or with other alternative and / or additional methods, materials, or components. In other instances, well-known structural, material, or operational details have not been shown or described to avoid obscuring aspects of the various embodiments of the invention. Similarly, for purposes of explanation, specific numbers, materials, and configurations are set forth to provide a thorough understanding of the invention. However, the invention may be practiced without the specific details. Furthermore, it should be understood that the various embodiments shown in the figures are illustrative representations and are not necessarily drawn to scale.

[0063] Further variations and alternative embodiments of the described systems and methods will be apparent to those skilled in the art from the description herein. Accordingly, it will be appreciated that the described systems and methods are not limited to these exemplary configurations. It should be understood that the forms of the systems and methods shown and described herein are to be considered exemplary embodiments. Various modifications to implementations may be made. Thus, while the invention is described herein with reference to specific embodiments, various modifications and changes can be made without departing from the scope of the invention. Accordingly, the specification and accompanying drawings should be regarded in an illustrative rather than a restrictive sense, and such modifications are intended to be included within the scope of the invention. Furthermore, any benefits, advantages, or solutions to problems described herein with respect to particular embodiments are not intended to be construed as critical, necessary, or essential features or elements of any or all of the claims.

Claims

1. 1. A method for selectively etching silicon nitride over oxide, comprising: providing a substrate having a silicon nitride layer and an oxide layer exposed on a surface thereof; exposing the surface of the substrate to a hydrogen plasma to (a) modify the exposed surface of the silicon nitride layer to form a first modified layer, and (b) modify the exposed surface of the oxide layer to form a second modified layer, wherein the gas pressure used to generate the hydrogen plasma and the temperature of the substrate form a crystallized water layer that combines with the oxide layer to form the second modified layer; selectively etching the silicon nitride layer by exposing the surface of the substrate to a halogen plasma to remove the first modified layer without removing the second modified layer; repeating the steps of exposing the surface of the substrate to the hydrogen plasma and exposing the surface of the substrate to the halogen plasma one or more times until a predetermined amount of the silicon nitride layer is selectively etched; A method comprising:

2. 2. The method of claim 1, wherein during the step of exposing the surface of the substrate to the halogen plasma, the crystallized water layer reduces etching of the oxide layer by inhibiting reactive species of the halogen plasma from reaching the second modified layer.

3. the first modified layer is a silicon layer and the second modified layer is a silicon dioxide layer; 3. The method of claim 2, wherein reactive species of the halogen plasma selectively etch silicon over silicon dioxide, further inhibiting or stopping etching of the oxide layer.

4. 2. The method of claim 1, wherein the gas pressure is in the range between 10 mTorr and 1500 Torr and the temperature of the substrate is in the range between 0°C and -80°C.

5. 2. The method of claim 1 , wherein during the step of exposing the surface of the substrate to the hydrogen plasma, the hydrogen plasma reacts with the exposed surface of the silicon nitride layer to form a first reaction by-product that is vaporized at the gas pressure and the temperature of the substrate to form the first modified layer.

6. The first reaction by-product is ammonia (NH ３ 6. The method of claim 5, wherein the first modified layer is a silicon layer.

7. during the step of exposing the surface of the substrate to the hydrogen plasma, the hydrogen plasma reacts with the exposed surface of the oxide layer to generate a second reaction by-product; 10. The method of claim 1, wherein the second reaction by-product freezes at the gas pressure and the temperature of the substrate to form the crystallized water layer on the oxide layer, forming the second modified layer.

8. The second reaction by-product is water (H ２ 8. The method of claim 7, wherein the second modified layer is a silicon dioxide layer.

9. the first modified layer is a silicon layer and the second modified layer is a silicon dioxide layer; 10. The method of claim 1, further comprising generating the halogen plasma from one or more process gases having an etch selectivity for silicon to silicon dioxide of 5:1 or greater.

10. The one or more process gases may include chlorine (Cl ２ ), hydrogen bromide (HBr), difluoride (F ２ ), xenon difluoride (XeF ２ ), tetrafluoromethane (CF ４ ), nitrogen trifluoride (NF ３ ), and sulfur hexafluoride (SF ６ 10. The method of claim 9, comprising one or more of:

11. 1. A method for selectively etching silicon nitride over oxide, comprising: providing a substrate having a silicon nitride layer and an oxide layer exposed on a surface thereof; generating a hydrogen plasma at a gas pressure of less than 1500 Torr and a temperature of the substrate of 0° C. or less; exposing the surface of the substrate to the hydrogen plasma, the hydrogen plasma comprising: reacting with the exposed surface of the silicon nitride layer to form a first reaction by-product, which is vaporized at the gas pressure and the temperature of the substrate to form a first modified layer on the silicon nitride layer; reacting with the exposed surface of the oxide layer to produce a second reaction by-product, which freezes at the gas pressure and the temperature of the substrate to form a layer of crystallized water, forming a second modified layer on the oxide layer; exposing the surface of the substrate to a halogen plasma to selectively etch the silicon nitride layer by removing the first modified layer without removing the second modified layer; A method comprising:

12. 12. The method of claim 11, further comprising repeating the steps of exposing the surface of the substrate to the hydrogen plasma and exposing the surface of the substrate to the halogen plasma for a plurality of cycles, wherein a predetermined amount of the silicon nitride layer is selectively etched.

13. 12. The method of claim 11, wherein the gas pressure is in the range between 10 mTorr and 1500 Torr and the temperature of the substrate is in the range between 0°C and -80°C.

14. 12. The method of claim 11, wherein during the step of exposing the surface of the substrate to the halogen plasma, the crystallized water layer reduces etching of the oxide layer by inhibiting reactive species of the halogen plasma from reaching the second modified layer.

15. the first modified layer is a silicon layer and the second modified layer is a silicon dioxide layer; 15. The method of claim 14, wherein the reactive species of the halogen plasma selectively etches silicon over silicon dioxide, further inhibiting or stopping etching of the oxide layer.

16. 12. The method of claim 11, wherein the halogen plasma is generated from one or more process gases having an etch selectivity of silicon to silicon dioxide of 5:1 or greater.

17. The hydrogen plasma is hydrogen (H ２ ), hydrogen bromide (HBr), methane (CH ４ ), and hydrogen sulfide (H ２ S), The halogen plasma is chlorine (Cl ２ ), hydrogen bromide (HBr), difluoride (F ２ ), xenon difluoride (XeF ２ ), tetrafluoromethane (CF ４ ), nitrogen trifluoride (NF ３ ), and sulfur hexafluoride (SF ６ 12. The method of claim 11 , wherein the carbon dioxide is produced from one or more process gases comprising:

18. The first reaction by-product is ammonia (NH ３ ), the first modified layer is a silicon layer, and the second reaction by-product is water (H ２ 12. The method of claim 11, wherein the second modified layer is a silicon dioxide layer.

19. When the surface of the substrate is exposed to the hydrogen plasma, the gas pressure used to generate the hydrogen plasma and the temperature of the substrate result in: The ammonia (NH ３ ) is vaporized to form the silicon layer; The water (H ２ 19. The method of claim 18, wherein O) freezes to form the crystallized water layer, which combines with the exposed surface of the oxide layer to form the silicon dioxide layer.

20. 20. The method of claim 19, wherein the gas pressure used to generate the hydrogen plasma and the temperature of the substrate are selected from a pressure-temperature window comprising a pressure range between 10 mTorr and 1500 Torr and a temperature range between 0°C and -80°C.

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