Patent Publication Number: US-2022216064-A1

Title: Plasma-assisted etching of metal oxides

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional application of U.S. patent application Ser. No. 16/944,653, filed on Jul. 31, 2020, titled “Plasma-Assisted. Etching of Metal Oxides,” the entire content of which is incorporated herein by reference. 
    
    
     BACKGROUND 
     Dry etching is a semiconductor manufacturing process that removes a masked pattern of material by exposing the material to a bombardment of ions. Before etching, a wafer is coated with photoresist or a hard mask (e.g., oxide or nitride) and exposed to a circuit pattern during a photolithography operation, Etching removes material from the pattern traces. This sequence of patterning and etching can be repeated multiple times during the semiconductor manufacturing process. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. 
         FIG. 1  illustrates a cross-sectional view of an exemplary plasma-assisted thermal atomic layer etching (ALE) system, in accordance with some embodiments. 
         FIG. 2  illustrates a cross-sectional view of another exemplary plasma-assisted thermal ALE system, in accordance with some embodiments. 
         FIGS. 3A and 3B  illustrate cross-sectional views of an exemplary plasma-assisted thermal ALE system with two chambers, in accordance with some embodiments. 
         FIGS. 4A and 4B  illustrate a surface modification cycle and a ligand exchange cycle respectively of an exemplary plasma-assisted thermal ALE process, in accordance with some embodiments. 
         FIG. 5  illustrates a thickness of a metal oxide layer changing with regard to cycle numbers for an exemplary thermal ALE process, in accordance with some embodiments. 
         FIGS. 6A and 6B  illustrate vertical and horizontal etching rates and a ratio of the vertical etching rate to the horizontal etching rate with respect to time of an exemplary plasma-assisted thermal ALE process, in accordance with some embodiments. 
         FIG. 7  illustrates a method of plasma-assisted thermal ALE of a metal oxide, in accordance with sonic embodiments. 
         FIGS. 8A and 8B  illustrates exemplary semiconductor devices with metal oxides, in accordance with some embodiments. 
         FIGS. 9A and 9B  illustrates exemplary semiconductor devices with metal oxides after a plasma-assisted thermal ALE process, in accordance with some embodiments. 
     
    
    
     Illustrative embodiments will now he described with reference to the accompanying drawings. In the drawings, like reference numerals generally indicate identical, functionally similar, and/or structurally similar elements. 
     DETAILED DESCRIPTION 
     The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. As used herein, the formation of a first feature on a second feature means the first feature is formed in direct contact with the second feature. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition does not in itself dictate a relationship between the various embodiments and/or configurations discussed. 
     Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper,” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or featur(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly, 
     It is noted that references in the specification to “one embodiment,” “an embodiment,” “an example embodiment,” “exemplary,” etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases do not necessarily refer to the same embodiment. Further, when a particular feature, structure or characteristic is described in connection with an embodiment, it would be within the knowledge of one skilled in the art to effect such feature, structure or characteristic in connection with other embodiments whether or not explicitly described. 
     It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation, such that the terminology or phraseology of the present specification is to be interpreted by those skilled in relevant art(s) in light of the teachings herein. 
     In some embodiments, the terms “about” and “substantially” can indicate a value of a given quantity that varies within 5% of the value (e.g., ±1%, ±2%, ±3%, ±4, ±5% of the value). These values are merely examples and are not intended to be limiting. The terms “about” and “substantially” can refer to a percentage of the values as interpreted by those skilled in relevant art(s) in light of the teachings herein. 
     Dry etching is a frequently used process in semiconductor manufacturing. Before etching, a water is coated with a photoresist or a hard mask (e.g., oxide or nitride) and a circuit pattern is transferred on the photoresist or the hard mask using photolithographic processes (e.g., photo exposure, post exposure bake, develop, hard bake, etc.). Etching is subsequently used to remove material from the surface of the wafer that is not covered by the patterned photoresist or hard mask. This sequence of patterning and etching can be repeated multiple times during chip manufacturing. 
     Plasma etching is performed by applying electromagnetic energy (e.g., radio frequency (RF)) to a gas that contains a chemically reactive element, such as nitrogen trifluoride and hydrogen, to form a plasma. The plasma releases positively charged ions that can bombard the surface of a wafer to remove, or etch, material. At the same time, chemically reactive radicals (e.g., atoms or groups of atoms with unpaired electrons) can react with the surface of the wafer to modify, surface properties. To improve etch throughput, higher etch rates (e.g., several A/min or nm/min) are desirable. 
     Process chemistries can differ depending on the types of films to be etched. For example, etch chemistries used in dielectric etch applications can be fluorine-based. Silicon and metal etch applications can use chlorine-based chemistries. An etch step may include etching one or more film layers from the surface of a wafer. When multiple layers are on the surface of the wafer, for example during the removal of a metal oxide, the etch process is required to remove the metal oxide hut preserve other layers (e.g., Si, silicon oxide, silicon nitride, etc.), the selectivity of the etch process becomes an important parameter. Selectivity of an etch chemistry or an etch process can be defined as the ratio of two etch rates: the rate for the layer to be removed to the rate for the layer to be preserved. In an etch process, high selectivity ratios (e.g., greater than 10:1) are desirable. The ions in the plasma etching can have higher kinetic energies than the radicals. As such, the ions can have a higher etch rate than the radicals. However, the ions can have a lower etch selectivity than the radicals. The term “etch selectivity” can refer to the ratio of the etch rates of two different materials under the same etching conditions. Higher etch rate with higher etch selectivity is an objective in an etch process. 
     In an ideal case, the etch rate of an etch chemistry is the same (uniform) at all points/locations on a wafer, or within a die on a wafer. For example, in such an ideal case, the etch chemistry can etch the same structure (e.g., remove a metal oxide) across the wafer the same way, or etch different structures (e.g. remove one or more structures of a metal oxide), within a die the same way. The degree to which the etch rate of an etch chemistry varies at different points/locations on the wafer, or within a die on a wafer, is known as non-uniformity. Improving uniformity is another objective in an etch process. 
     Various embodiments of the present disclosure provide an exemplary plasma-assisted thermal atomic layer etching (ALE) process. In some embodiments, the plasma-assisted thermal ALE process can increase an etch rate of a metal oxide layer on a wafer while maintaining etch selectivity between the metal oxide and adjacent materials on the wafer. The metal oxide can include hafnium oxide, aluminum oxide, zirconium oxide, and other suitable metal oxide dielectric materials. 
     Atomic layer etching, or ALE, is a technique that can remove thin layers of material from the surface of a wafer using sequential reaction cycles (e.g., duty cycles); for example, during the removal of a metal oxide on one or more dielectric layers. The sequential reaction cycles of an ALE process can be “quasi self-limiting.” In some embodiments, quasi self-limiting reactions may refer to those reactions that slow down as a function of time (e.g., asymptotically), or as a function of species dosage. A plasma-assisted thermal ALE process can include three sequential reaction cycles: (i) a surface modification cycle, (ii) a material removal cycle, and (iii) a surface cleaning cycle. The surface modification cycle can form a reactive surface layer with a defined thickness from a material on the surface of a wafer that has been exposed to the surface modification process. The modified material layer (reactive surface layer) can be subsequently removed during the next cycle (e.g., material removal cycle). Any unmodified material, which is not exposed to the surface modification chemistry during the surface modification cycle, will not be removed. The modified material, for example, can have a gradient in chemical composition and/or physical structure. The material removal cycle can remove the modified material layer while keeping the unmodified material(s) or layers intact. The total amount of material removed can be controlled by the number of repeated. cycles (e.g., surface modification cycle, material removal cycle, and surface cleaning cycle). The surface cleaning cycle can remove surface residues and byproducts from the material removal cycle on the surface of the wafer and reset the surface to a near-pristine state for the next etching cycle. 
     In some embodiments, a time elapsed between sequential cycles (e.g., between the surface modification cycle and the material removal cycle) is referred to as a “transition time.” During the transition time, reactants/byproducts from a current cycle are removed away from the surface of the wafer, prior to the release of new reactants. Prompt delivery of reactants into the chamber can reduce the transition time between cycles and the cycle duration (cycle time). 
     The plasma assisted thermal ALE technique can be used in a variety of etching schemes including, but not limited to, directional or isotropic etching (e.g., formation of air spacers) and selective or nonselective etching (e.g., removal of dielectric layers from an exposed surface). In a plasma assisted thermal ALE process the reactants can be, for example, delivered by one or more gases, a plasma, a vapor, or other suitable sources. 
     In some embodiments, the plasma-assisted thermal ALE process can modify the surface of the metal oxide layer with radicals from a plasma during the surface modification cycle. The material removal cycle can include a ligand exchange reaction, which can be performed under a thermal condition. In some embodiments, radicals of a plasma can increase the ligand exchange kinetic energy and the speed of the ligand exchange reaction, thus increasing removal of the modified surface of the metal oxide layer and the etching rate of the metal oxide layer. In some embodiments, one or more plates with evenly distributed holes or openings can distribute the gases and plasmas uniformly across the wafer. In some embodiments, a plasma flush of radicals during the surface cleaning cycle can remove surface ligand residues and byproducts and create a fresh surface for the next etching cycle. The plasma flush can further increase the etching rate of the plasma-assisted thermal ALE process. 
       FIG. 1  illustrates a cross-sectional view of an exemplary plasma-assisted thermal atomic layer etching (ALE) system  100 , in accordance with some embodiments. By way of example and not limitation, plasma-assisted thermal ALE system  100  can include a chamber  102 , a shower head  103  and a wafer holder  104  in chamber  102 , a first gas line  106  and a second gas line  108  connected to chamber  102 , and a plasma generator  110  connected to wafer holder  104 . In some embodiments, an inner surface of chamber  102  can be covered with yttrium oxide (Y 2 O 3 ) to protect chamber  102  from the plasmas and etch chemistries in the plasma-assisted ALE process. Shower head  103  can connect to first gas line  106  and release gases from first gas line  106  into chamber  102 . A pressure in chamber  102  can range from about 3 mTorr to about 4 Torr. In the surface modification cycle, the pressure in chamber  102  can range from about 1 Torr to about 4 Torr. If the pressure is less than about 1 Torr, a ratio of the ions to the radicals in the plasma can be too high to cause surface damage. If the pressure is greater than about 4 Torr, the plasma may not be formed to assist the thermal ALE process. In the material removal cycle, the pressure in chamber  102  can range from about 3 mTorr to about 1000 mTorr. If the pressure is less than about 3 mTorr, a ratio of the ions to the radicals in the plasma can be too high to cause surface damage, and the ligand exchange precursors may be decomposed. If the pressure is greater than about 1000 mTorr, the ligand exchange precursors may be condensed. In the surface cleaning cycle, the pressure in chamber  102  can range from about 20 mTorr to about 200 mTorr. If the pressure is less than about 20 mTorr, a ratio of the ions to the radicals in the plasma can be too high to cause surface damage. If the pressure is greater than about 1000 mTorr, the plasma may not be formed to assist the thermal ALE process. 
     Wafer holder  104  can be an electrostatic wafer chuck and configured to hold a wafer  112 . Wafer  112  can be patterned and have areas of a metal oxide layer on a surface of wafer  112  exposed for etching. In some embodiments, the metal oxide layer can include hafnium oxide, aluminum oxide, zirconium oxide, and other suitable metal oxide dielectric materials. Wafer holder  104  can include a heater (not shown) to heat wafer  112 . In some embodiments, wafer  112  can be heated to a temperature ranging from about 150° C. to about 350° C. for the plasma-assisted thermal ALE process. If the temperature is less than about 150° C., the ligand exchange reaction may not be performed and the metal oxide layer may not be removed. If the temperature is greater than about 350° C., the plasma-assisted thermal ALE process may have no etch selectivity between the metal oxide layer and adjacent structures and cause surface damage. In some embodiments, plasma generator  110  can connect to wafer holder  104 , apply a radio frequency (RF) signal to wafer holder  104 , and generate a plasma in chamber  102 . 
     First gas line  106  can include a first valve  114  controlling a gas flow of first gas  120  and a second valve  116  controlling a gas flow of second gas  122 . In some embodiments, first gas  120  and second gas  122  can be delivered from a gas cabinet (not shown). In some embodiments, first gas  120  can include one or more surface modification gases, such as hydrogen fluoride (HF) and nitrogen trifluoride (NF 3 ). Second gas  122  can include a surface cleaning gas, such as hydrogen and argon. In some embodiments, first gas  120  can include a plasma of the surface modification gases and second gas  122  can include a plasma of the surface cleaning gas. A remote plasma generator (not shown) can generate the plasma of the surface modification gases and the plasma of the surface cleaning gas. First gas line  106  can direct the plasma of the surface modification gases and the plasma of the surface cleaning gas to shower head  103  in chamber  102 . In some embodiments, second gas  122  can include a cleaning gas (e.g., helium) for a transition cycle after each cycle of the plasma-assisted thermal ALE process to pump and purge chamber  102  to prevent intermixing of gases and plasmas. In some embodiments, the transition cycle can last from about 30 s to about 60 s. 
     Second gas line  108  can include a third valve  118  controlling a gas flow of a vapor  124  flowing from a vaporizer  115  into chamber  102 . Vaporizer  115  can convert a ligand exchange precursor from liquid to vapor  124 , which can be drawn to chamber  102  by the vacuum in chamber  102 . In some embodiments, a flow rate of vapor  124  can range from about 50 sccm to about 900 sccm. If the flow rate of vapor  124  is less than about 50 sccm, the modified surface may not be fully removed. If the flow rate of vapor  124  is greater than about 900 sccm, ligand residues may form on the surface of wafer  112 . 
     Plasma-assisted thermal ALE system  100  can further include a first plate  126 , a second plate  130 , and a third plate  132  in chamber  102 . In some embodiments, first plate  126  can have evenly distributed openings or concentric openings to uniformly distribute first gas  120  and second gas  122  delivered into chamber  102 . Plasma region  128  can be formed between first plate  126  and second plate  130  by plasma generator  110 . When first valve  114  is open and first gas  120  is delivered to chamber  102 , plasma region  128  can include ions and radicals of first gas  120 . When second valve  116  is open and second gas  122  is delivered to chamber  102 , plasma region  128  can include ions and radicals of second gas  122 . In some embodiments, second plate  130  can have evenly distributed openings or concentric openings similar to first plate  126 . In some embodiments, second plate  130  can be electrically connected to an external power supply (not shown), such as a direct current (DC) power supply that keeps second plate  130  at a negative bias voltage ranging from about −1 Volt to about −500 Volts, to filter out ions. Radicals in plasma region  128  can pass through second plate  130 . In some embodiments, second plate  130  can be electrically connected to a ground acting as a discharger for the ions. Second plate  130  can neutralize ions and form radicals with higher kinetic energies than radicals generated in plasma region  128 . In some embodiments, third plate  132  can connect to second gas line  108  and have evenly distributed openings or nozzles on the side of third plate  132  facing wafer  112 . Third plate  132  can generate uniformly distributed vapor  124  of ligand exchange precursor in gas region  134  around the surface of water  112 . Uniformly distributed vapor of ligand exchange precursor in gas region  134  can improve the uniformity of the ligand exchange reaction on the surface of wafer  112  and the uniformity of etching profiles across wafer  112 . 
       FIG. 2  illustrates a cross-sectional view of another exemplary plasma-assisted thermal ALE system  200 , in accordance with some embodiments. As shown in  FIG. 2 , plasma-assisted thermal ALE system  200  can include a chamber  202 , shower head  103  and wafer holder  204  in chamber  202 , plasma generator  110  connected to wafer holder  204 , first gas line  106 , second gas line  108 , and third gas line  206 . Elements in  FIG. 2  with the same annotations as elements in  FIG. 1  are described above. A pressure in chamber  202  can range from about 1 mTorr to about 500 mTorr. Wafer holder  204  can be an electrostatic wafer chuck and configured to hold and heat wafer  112 , similar to wafer holder  104 . 
     Third gas line  206  can include second valve  116  controlling a gas flow of second gas  122 . Different from plasma-assisted thermal ALE system  100 , plasma-assisted thermal ALE system  200  can deliver second gas  122  to wafer  112  using third gas line  206  separate from first gas  120  (e.g., on sidewalk of chamber  202 ). In some embodiments, without a gas distribution plate, third gas line  206  can improve process control of distributing second gas  122  uniformly on wafer  112  and can improve surface cleaning after the material removal cycle. 
     Gas region  234  can include a plasma of first gas  120  during the surface modification cycle, vapor  124  of ligand exchange precursor during the material removal cycle, and a plasma of second gas  122  during the surface cleaning cycle, according to some embodiments. Plasma generator  110  can generate a plasma of first gas  120  and a plasma of second gas  122  in gas region  234  during the plasma-assisted thermal ALE process. Vapor  124  of ligand exchange precursor can be delivered to gas region  234  by second gas line  108 . In some embodiments, comparing plasma-assisted thermal ALE systems  100  and  200 , plasma-assisted thermal ALE system  100  can have plasmas and precursors more uniformly distributed in gas region  134  with first plate  126 , second plate  130 , and third plate  132 , while ALE system  200  can have an easier design. 
       FIGS. 3A and 3B  illustrate cross-sectional views of an exemplary plasma-assisted. thermal ALE system  300  with chamber  302 A and chamber  302 B, in accordance with some embodiments. As shown in  FIGS. 3A and 3B , plasma-assisted thermal ALE system  300  can include chamber  302 A, shower head  103  and a wafer holder  304 A in chamber  302 A, plasma generator  110  connected to wafer holder  304 A, first gas line  106  and third gas line  206  connected to shower head  103 . Plasma-assisted thermal ALE system  300  can further include chamber  302 B, a wafer holder  304 B in chamber  302 B, second gas line  108  connected to chamber  302 B. Chamber  302 A and chamber  302 B can be connected by connector  336 , which can be configured to connect chamber  302 A and  302 B and transfer wafer  112  between chamber  302 A and chamber  302 B without breaking a vacuum. Elements in  FIGS. 3A and 3B  with the same annotations as elements in  FIGS. 1 and 2  are described above. The pressures in chamber  302 A and  302 B can range from about 1 mTorr to about 500 mTorr. Wafer holders  304 A and  304 B can be electrostatic wafer chucks and configured to hold and heat wafer  112 , similar to wafer holder  104 . 
     According to some embodiments, the plasma-assisted thermal ALE process can have the surface modification cycle and the cleaning cycle in chamber  302 A and the material removal cycle in chamber  302 B. As shown in  FIGS. 3A and 3B , plasma generator  110  in chamber  302 A can generate a plasma of first gas  120  in plasma region  328  during the surface modification cycle. After the surface modification cycle, wafer  112  can be transferred to chamber  302 B through connector  336 , Vapor  124  of the ligand exchange precursor can be delivered to chamber  302 B via second gas line  108 . A plate  332  can be connected to second gas line  108  and can have evenly distributed openings or nozzles similar to third plate  132  on the side facing water  112 . Plate  332  can distribute vapor  124  uniformly in gas region  334  around wafer  112  to improve the uniformity of the ligand exchange reaction on the surface of wafer  112 . After the material removal cycle, water  112  can be transferred. back to chamber  302 A through connector  336  for the surface cleaning cycle. Plasma generator  110  in chamber  302 A can generate a plasma of second gas  122  in plasma region  328  and clean the surfaces of the metal oxide layer on wafer  112  with the plasma. Comparing plasma-assisted thermal ALE systems  100  and  300 , plasma-assisted thermal ALE system  1100  can have plasma-enhanced ligand exchange reaction during the material removal cycle. As ALE system  300  can have separate chambers  302 A and  302 B for plasmas and ligand exchange precursors respectively, ALE system  300  may not need transition cycles after each cycle of the plasma-assisted thermal ALE process, which can reduce process time and improve process control. 
       FIGS. 4A and 4B  illustrate a surface modification cycle and a ligand exchange cycle respectively of an exemplary plasma-assisted thermal ALE process, in accordance with some embodiments. By way of example and not limitation, a surface of a metal oxide layer  338  can be fluorinated by fluorine radicals generated from the plasma of first gas  120  by plasma generator  110 , as shown in  FIG. 4A . In some embodiments, metal oxide layer  338  can include aluminum oxide and first gas  120  can include NF 3 . In some embodiments, the plasma of first gas  120  can be generated at a pressure ranging from about 1 Torr to about 4 Torr with a power ranging from about 400 W to about 700 W. The gas flow rate of first gas  120  can range from about 100 sccm to about 500 sccm. A temperature of the plasma process can range from about 250° C. to about 300° C. A time of the surface modification cycle can range from about 10 s to about 30 s and a depth  338   d  of fluorinated metal oxide on the surface of metal oxide layer  338  can range from about 3 Å to about 10 Å after the surface modification cycle. lf depth  338   d  is less than about 3 Å, the surface of metal oxide layer  338  may not be fully fluorinated for the ligand exchange reaction. If depth  338   d  is greater than about 10 Å, ligand residues may be formed after the ligand exchange reaction. During the surface modification cycle, water vapor (H 2 O) and/or methane (CH 4 ) can be formed and removed by the vacuum in the plasma-assisted thermal ALE system. 
     The surface modification cycle can be followed by the material removal cycle, as shown in  FIG. 4B . By way of example and not limitation, a ligand exchange precursor for aluminum oxide can include diethylaluminium chloride (C 4 H 10 AlCl or DMAC) and react with the fluorinated surface of metal oxide layer  338 . The fluorinated metal oxide can be removed from metal oxide layer  338  and ligand residues and byproducts can remain on the surface of metal oxide layer  338 . In some embodiments, the ligand exchange reaction can be performed at a temperature ranging from about 250° C. to about 300 20   C. In some embodiments, the ligand exchange reaction can be accelerated by higher energy radicals generated by third plate  132  from the plasma of second gas  122  (shown in  FIG. 1 ). The plasma of second gas  122  can be generated by plasma generator  110  at a pressure ranging from about 100 mTorr to about 1000 mTorr with a power ranging from about 250 W to about 400 W. In some embodiments, plasma generator  110  can use pulsing power with a duty cycle ranging from about 10% to about 70%, which means the power of plasma generator  110  can be on for about 10% to about 70% of the time during the material removal cycle. The gas flow rate of second gas  122  can range from about 1000 sccm to about 5000 sccm. In some embodiments, second gas  122  can include hydrogen or argon to provide higher energy radicals for the ligand exchange reaction. In some embodiments, the flow rate of vapor  124  of ligand exchange precursor can range from about 50 sccm to about 900 sccm. The time to remove the fluorinated surface of metal oxide layer  338  can range from about 10 s to about 50 s. After the material removal cycle, the fluorinated metal oxide on the surface of metal oxide layer  338  can be removed and a thickness of the removed metal oxide can range from about 3 A to about 10 A, the same as depth  338   d.    
     The material removal cycle can be followed by surface cleaning cycle in the plasma-assisted thermal ALE process (not shown). By way of example and not limitation, second gas  122  can include a surface cleaning gas, such as hydrogen. Plasma generator  110  can generate a plasma of the surface cleaning gas. Radicals of the plasma of second gas  122  can clean the surface of metal oxide layer  338 , remove about 90% to about 100% of the ligand exchange residues and byproducts, and reset the surface to a condition with substantially no residue for the next etching cycle. In some embodiments, additional surface cleaning may be needed to remove the ligand exchange residues and byproducts on the surface. In some embodiments, the plasma of second gas  122  can be generated at a pressure ranging from about 20 mTorr to about 200 mTorr with a power ranging from about 100 W to about 400 W. The gas flow rate of second gas  122  can range from about 100 sccm to about 1000 sccm. A temperature of the plasma process can range from about 250° C. to about 300° C. A time of the surface cleaning cycle can range from about 10 s to about 30 s. If the time is less than about 10 s, ligand residues and byproducts may not be fully removed from the surface of metal oxide layer  338 . The ligand residues and byproducts can block surface fluorination of the surface modification cycle. If the time is greater than about 30 s, exposed areas of other materials (e.g., silicon oxide, silicon nitride, silicon, etc.) may be damaged. 
       FIG. 5  illustrates a thickness of a metal oxide layer changing with regard to cycle numbers for an exemplary thermal ALE process, in accordance with some embodiments. Embodiment 1 can include surface modification cycles and material removal cycles without plasma assistance, and embodiment 2 can include surface modification cycles, material removal cycles, and surface cleaning cycles with plasma assistance. A slope of the thickness with regard to the cycle numbers for each embodiment represented respective etching rate of the metal oxide layer. As shown in  FIG. 5 , embodiment 2 can have a higher etching rate than embodiment 1 because of the surface cleaning cycle and plasma assistance. In some embodiments, an etching rate of embodiment 1 can range from about 0.1 A/cycle to about 0.5 A/cycle. In some embodiments, an etching rate of embodiment 2 can range from about 5 A/cycle to about 10 A/cycle. In some embodiments, a ratio of the etching rate of embodiment 1 to embodiment 2 can range from about 10 to about 100. 
       FIGS. 6A and 6B  illustrate vertical and horizontal etching rates and a ratio of the vertical etching rate to the horizontal etching rate with respect to time of an exemplary plasma-assisted thermal ALE process, in accordance with some embodiments. As shown in  FIGS. 6A and 6B , the plasma-assisted thermal ALE process can have a vertical etching rate higher than a horizontal etching rate. The vertical etching rate can saturate with the increase of etching time while the horizontal etching rate can gradually increase with the increase of etching time. As a result, a ratio of the horizontal etching rate to the vertical etching rate (also referred to as “isotropy factor”) can increase with the etching time. For example, as shown in  FIGS. 6A and 6B , vertical etching rate is higher than horizontal etching rate at t 1 , t 2 , and t 3 . Vertical etching rate can saturate at t 4  and t 5  while horizontal etching rate can still increase. The change of the ratio of horizontal etching rate to vertical etching rate with time can affect the etching profile of the metal oxide layer. For example, if a vertical profile is desired, such as removing a sacrificial metal oxide layer and forming an air spacer, the etching time per etching cycle can be controlled shorter than t 3 . If a horizontal etching is desired, such as removing a gate dielectric layer of a metal oxide, the etching time per etching cycle can be controlled longer than t 5 . 
       FIG. 7  illustrates a flow diagram of method  700  for plasma-assisted thermal ALE of a metal oxide, in accordance with some embodiments. Additional operations may be performed between various operations of method  700  and may be omitted merely for clarity and ease of description. Additional operations can be provided before, during, and/or after method  700 ; one or more of these additional processes are briefly described herein. Therefore, method  700  may not be limited to the operations described below. 
     Method  700  can be performed by exemplary plasma-assisted thermal ALE systems  100 ,  200 , and  300  shown in  FIGS. 1, 2, and 3A and 3B . For illustrative purposes, the operations in  FIG. 7  will be described with reference to exemplary plasma-assisted thermal ALE system  100  shown in  FIG. 1  and the exemplary plasma-assisted thermal ALE process in  FIGS. 4A and 4B . As shown in  FIG. 1 , plasma-assisted thermal ALE system  100  can include first gas line  106  to deliver first gas  120  and second gas  122  to chamber  102  and second gas line  108  to deliver vapor  124  to chamber  102 . Shower head  103  can release gases from first gas line  106  to chamber  102 . Wafer holder  104  can hold and heat wafer  112  having metal oxide layer  338  on the surface exposed for etching. Plasma generator  110  can generate plasmas from first gas  120  and second gas  122 . 
     Referring to  FIG. 7 , method  700  begins with operation  710  and the process of modifying a surface of a metal oxide with a first gas. As shown in  FIG. 1 , first valve  114  can open and first gas  120  can be delivered to chamber  102 . In some embodiments, first gas  120  can include one or more surface modification gases, such as HF and NF 3 . In some embodiments, first gas  120  can include a plasma of the one or more surface modification gases generated from a remote plasma generator (not shown). First plate  126  can have evenly distributed openings or concentric openings to uniformly distribute first gas  120  over wafer  112 . Plasma generator  110  can generate a plasma of first gas  120  and form plasma region  128  between first plate  126  and second plate  130 . Plasma region  128  can include ions and radicals of the plasma of first gas  120 . In some embodiments, second plate  130  can be biased at a negative voltage ranging from about −1 Volt to about −500 Volts to filter out ions. Radicals in plasma region  128  can pass through second plate  130  and reach the surface of metal oxide layer  338  on wafer  112 . 
     In some embodiments, surface modification refers to a process where the radicals of first gas  120  (e.g., NF 3 ) interacts with the exposed materials on the surface of metal oxide layer  338  on wafer  112  and forms a reactive surface layer or modified material layer with a defined thickness. The modified material layer can be subsequently removed during the removal, or etch, cycle. Any unmodified material, which is not exposed to the radicals of first gas  120  during the surface modification cycle, will not be removed. The modified material can include a gradient in chemical composition and/or physical structure. In some embodiments, the surface modification cycle can have a duration from about 10 s to about 30 s and the modified metal oxide layer can have depth  338   d  ranging from about 3 Å to about 10 Å (shown in  FIG. 4A ). However, the surface modification cycle can be shorter or longer, and may depend on the geometry of chamber  102  (e.g., the volume, the distance of shower head  103  from wafer  112 , etc.), the pumping speed of the pump stack (not shown in  FIG. 1 ), or other process parameters (e.g., self-limiting behavior of first gas  120 , etc.). 
     In some embodiments, after the surface modification cycle, a transition cycle may be introduced to remove any unreacted quantities of first gas  120  in first gas line  106  and chamber  102 . During the transition cycle, the flow of first gas  120  can be stopped by first valve  114  and its partial pressure is reduced as it is pumped out of chamber  102 . In some embodiments, the transition cycle can including purging first gas line  106  and chamber  102  with an inert gas, such as helium. In some embodiments, the transition cycle can last from about 30 s to about 60 s. However, the transition cycle can be shorter or longer, and may depend on the geometry of chamber  102  (e.g., the volume, the distance of shower head  103  from wafer  112 , etc.), the pumping speed of the pump stack (not shown in  FIG. 1 ), or other process parameters. 
     Referring to  FIG. 7 , method  700  continues with operation  720  and the process of removing a top portion of the metal oxide by a ligand exchange reaction. As shown in  FIG. 1 , first valve  114  can be closed and third valve  118  can open. Vapor  124  of ligand exchange precursor can be delivered to chamber  102 . In some embodiments, third plate  132  can connect to second gas line  108  and generate uniformly distributed vapor  124  of ligand exchange precursor in gas region  134 . In some embodiments, second valve  116  can open and second gas  122  can be delivered to chamber  102  during the material removal cycle. Second gas  122  can include hydrogen or argon to provide higher energy radicals for the ligand exchange reaction. Plasma. generator  110  can generate a plasma of second gas  122  and form plasma region  128  between first plate  126  and second plate  130 . Plasma region  128  can include ions and radicals of the plasma of second gas  122 . In some embodiments, second plate  130  can be electrically connected to a ground acting as a discharger. Second plate  130  can neutralize ions and form radicals with higher kinetic energies than radicals generated in plasma region  128 . Higher kinetic energies radicals can accelerate the ligand exchange reaction and increase the etching rate of metal oxide layer  338 . In some embodiments, the material removal cycle can be performed at a temperature ranging from about 250° C. to about 300° C. The material removal cycle can remove a top portion of modified materials on wafer  112 , for example, fluorinated metal oxide on the surface of metal oxide layer  338  with a depth  338   d  as shown in  FIG. 4A . In some embodiments, depth  338   d  can range from about 3 A to about 10 A. In some embodiments, after the material removal cycle, another transition cycle as described above may be performed to remove any unreacted quantities of ligand exchange precursor in chamber  102 . 
     Referring to  FIG. 7 , method  700  continues with operation  730  and the process of cleaning the surface of the metal oxide with a plasma of a second gas. As shown in  FIG. 1 , first valve  114  and third valve  118  can be closed and second valve  116  can open. Second gas  122  can be delivered to chamber  102 . In some embodiments, second gas  122  can include a surface cleaning gas, such as hydrogen. In some embodiments, second gas  122  can include a plasma of the surface cleaning gas generated from a remote plasma generator (not shown). First plate  126  can distribute second gas  122  uniformly over wafer  112 . Plasma generator  110  can generate a plasma of second gas  122  and form plasma region  128  between first plate  126  and second plate  130 . Plasma region  128  can include ions and radicals of the plasma of second gas  122 . In some embodiments, second plate  130  can be biased at a negative voltage ranging from about −1 Volt to about −500 Volts to filter out ions. Radicals of second gas  122  in plasma region  128  can pass through second plate  130  and clean the surface of metal, oxide layer  338  on wafer  112 . The surface cleaning cycle can reset the surface of metal oxide layer  338  to a near-pristine state for the next etching cycle of plasma-assisted thermal ALE. 
       FIGS. 8A and 8B  illustrate exemplary semiconductor devices  850 A and  850 B respectively with metal oxides  884 , in accordance with some embodiments. Semiconductor devices  850 A and  850 B can include planar metal oxide semiconductor field-effect transistors (MOSFETs) or fin field effect transistors (finFETs). As shown in  FIGS. 8A and 8B , semiconductor devices  850 A and  850 B can both include fin structures  852 , dielectric liners  854 , dielectric layers  856 , source/drain (S/D) epitaxial structures  858 , gate structures  860 , and capping structures  878 . Gate structures  860  can include gate dielectric layers  862  and gate electrodes  864 . Gate electrodes  864  can include work function layers  866  and metal fills  868 . In some embodiments, semiconductor device  850 A can include an S/D contact structure  870 A connecting to S/D epitaxial structure  858 , as shown in  FIG. 8A . S/D contact structure  870 A can include a silicide layer  872 A, a metal liner  874 A, and a metal contact  876 A. In some embodiments, semiconductor device  850 B can include a dielectric plug  840 B on top of S/D epitaxial structure  858  and S/D epitaxial structure  858  may not be connected to an S/D contact structure, as shown in  FIG. 8B . 
     Semiconductor devices  850 A and  850 B can further include gate spacers  880 . Gate spacers  880  can include first dielectric layers  882 , sacrificial dielectric layers  884 , and second dielectric layers  886 . First dielectric layers  882  can include a dielectric material, such as silicon oxide, silicon nitride, a low-k material, and a combination thereof. The term “low-k” can refer to a small dielectric constant. In the field of semiconductor device structures and manufacturing processes, low-k can refer to a dielectric constant that is less than the dielectric constant of silicon oxide (e.g., less than about 3.9). Sacrificial dielectric layers  884  can include a metal oxide, such as aluminum oxide. Second dielectric layers  886  can include a dielectric material similar to first dielectric layers  882 . 
     In some embodiments, the plasma-assisted thermal ALE process described above (e.g., method  700  of  FIG. 7 ) can remove sacrificial dielectric layers  884  using exemplary plasma-assisted thermal ALE system  100 ,  200 , or  300  shown in  FIGS. 1, 2, 3A, and 3B  respectively. After the plasma-assisted thermal ALE process, sacrificial dielectric layers  884  can be removed and openings  984  can be formed between first dielectric layers  882  and second dielectric layers  886 , as shown in  FIGS. 9A and 9B . In some embodiments, the plasma-assisted thermal ALE process can increase the etch rate of the metal oxide in sacrificial dielectric layers  886  while maintaining etch selectivity between sacrificial dielectric layers  886  and adjacent first and second dielectric layers  882  and  886 . For example, the etch rate of sacrificial dielectric layers  886  can range from about 5 Å/cycle to about 10 Å/cycle. In some embodiments, after removing sacrificial dielectric layers  884  with the plasma-assisted thermal ALE process, openings  984  can have a horizontal dimension  984   w  (e.g., width) along an X-axis ranging from about 1 nm to about 4 nm and a vertical dimension  984   h  (e.g., height) along a Z-axis ranging from about 8 nm to about 16 nm. In some embodiments, a ratio of vertical dimension  984   h  to horizontal dimension  984   w  can range from about 2 to about 16. 
     In some embodiments, the removal of sacrificial dielectric layers  884  can be followed by formation of sealing structures to seal openings  984  and form air spacers (not shown) between gate structures  860  and adjacent structures S/D contact structures  870 A), which can reduce parasitic capacitance and improve device performance of semiconductor devices  850 A and  850 B. 
     Various embodiments of the present disclosure provide an exemplary plasma-assisted thermal atomic layer etching (ALE) process. In some embodiments, the plasma-assisted thermal ALE process can increase an etch rate of metal oxide layer  338  while maintaining etch selectivity between metal oxide layer  338  and adjacent materials on wafer  112 . A plasma-assisted thermal ALE process can include three sequential reaction cycles: (i) a surface modification cycle, a material removal cycle, and (iii) a surface cleaning cycle. In some embodiments, the plasma-assisted thermal ALE process can modify the surface of metal oxide layer  338  with radicals from a plasma during the surface modification cycle. The material removal cycle can include a ligand exchange reaction, which can be performed under a thermal condition. In some embodiments, radicals of a plasma can increase the ligand exchange kinetic energy and the speed of the ligand exchange reaction, thus increasing removal of the modified surface of metal oxide layer  338  and the etching rate of the metal oxide layer  338 . In some embodiments, plates  126 ,  130 , and  132  with evenly distributed openings or nozzles can distribute the plasmas and the gases uniformly across the wafer, In some embodiments, a plasma flush of radicals during the surface cleaning cycle can remove surface ligand residues and byproducts and create a fresh surface for the next etching cycle. The plasma flush can further increase the etching rate of the plasma-assisted thermal ALE process. 
     In some embodiments, a method for plasma-assisted etching of a metal oxide includes modifying a surface of the metal oxide with a first gas, removing a top portion of the metal oxide by a ligand exchange reaction, and cleaning the surface of the metal oxide with a second gas. 
     In some embodiments, a system for plasma-assisted etching of a metal oxide includes a wafer holder configured to hold a wafer with the metal oxide in a chamber, a first gas line connected to the chamber and configured to deliver a first gas and a second gas to the chamber, a second gas line connected to the chamber and configured to deliver a precursor to the chamber for a ligand exchange reaction on the metal oxide, and a plasma generator connected to the wafer holder and configured to generate a plasma of the first gas to modify a surface of the metal oxide and a plasma of the second gas to clean the surface of the metal oxide. 
     In some embodiments, a system for plasma-assisted etching of a metal oxide includes a chamber, a first gas line, and a second gas line. The chamber include a wafer holder configured to hold a wafer with the metal oxide and a plasma generator connected to the wafer holder and configured to generate a plasma from a first gas to modify the surface of the metal oxide and a second gas to clean the surface of the metal oxide. The first gas line is connected to the chamber and configured to deliver the first gas to the wafer. The second gas line is connected to the chamber and configured to deliver the second gas to the wafer. 
     It is to be appreciated that the Detailed Description section, and not. the Abstract of the Disclosure section, is intended to be used to interpret the claims. The Abstract of the Disclosure section may set forth one or more but not all possible embodiments of the present disclosure as contemplated by the inventor(s), and thus, are not intended to limit the subjoined claims in any way. 
     The foregoing disclosure outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art will appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art will also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure.