Method for etching of metal

Abstract

A metal etching method includes performing at least two cycles of an etching process. Each cycle of the etching process includes: performing surface modification on an exposed surface of a metal layer above a substrate, performing hydrogen treatment on the metal layer, and performing a cleaning process on the metal layer. The hydrogen treatment forms a layer of reaction products on the metal layer. The cleaning process removes the layer of reaction products.

Description

[Technical Field]

[0001] [Related Applications] This application claims priority to U.S. Provisional Patent Application No. 63 / 252,738, filed on October 6, 2021, entitled “Atomic Layer Etching of Copper”, the contents of which are incorporated herein by reference.

[0002] The present invention relates to the field of semiconductor manufacturing and semiconductor devices, and in a particular embodiment to a system and method for atomic layer etching of metals. [Previous Technology]

[0003] Selective dry etching of metals (e.g., copper (Cu)) at the atomic level can be used in many different applications in the semiconductor industry. For example, for 3D integration, hybrid bonding involving metal / metal bonding (e.g., Cu / Cu bonding) can be advantageous to produce proper and reliable electrical contacts with nanometer-level control over the depth of the metal pad recess. Although metal pad recesses can be achieved via chemical mechanical planarization (CMP) processes, this can become more difficult as feature sizes shrink and metal and dielectric surfaces are polished at the same rate. [Summary of the Invention]

[0004] According to one embodiment, a metal etching method includes: performing at least two cycles of an etching process, wherein each cycle includes performing surface modification on an exposed surface of a metal layer above a substrate; performing hydrogen treatment on the metal layer, wherein the hydrogen treatment forms a layer of reaction products on the metal layer; and performing a cleaning process on the metal layer, wherein the cleaning process removes the layer of reaction products.

[0005] According to another embodiment, a method for smoothing a surface includes: providing a substrate, a metal layer system located above the substrate, the metal layer system having a rough or oxidized surface; and performing one or more cycles of a smoothing process, wherein each cycle includes exposing the rough or oxidized surface to a plasma-excited H2 gas, wherein the plasma-excited H2 gas chemically reduces the oxidized surface of the metal layer and forms a layer of reaction products on the metal layer; and exposing the rough or oxidized surface to a plasma-excited Ar gas, wherein the plasma-excited Ar gas removes the layer of reaction products covering the metal layer, wherein the kinetic energy of the plasma-excited Ar gas is at or below a sputtering threshold for removing metal from the metal layer.

[0006] According to another embodiment, a plasma processing system includes: a chamber including a substrate support configured to support a substrate; a gas injection system coupled to the chamber; a vacuum pump system coupled to the chamber; and a computer coupled to the gas injection system and the vacuum pump system, wherein the computer is configured to execute a program including instructions to perform the following: expose a metal layer on the substrate to an oxidizing gas; expose the metal layer to a plasma-excited H2 gas; and expose the metal layer to a plasma-excited inert gas.

[0007] It should be understood that the foregoing general description and the following detailed description are merely exemplary and explanatory, and are not intended to limit the content of the disclosures claimed.

Implementation Method

[0020] The various embodiments will now be discussed in detail. However, it should be understood that the various embodiments described herein are applicable to a wide range of specific environments. The specific embodiments discussed are merely illustrative of specific ways of making and using the various embodiments and should not be construed as limiting the scope.

[0021] According to one or more embodiments of this disclosure, this application relates to a vapor phase atomic layer etching (ALE) method for metals (e.g., Cu). These methods provide selective etching of metals relative to other materials (e.g., dielectrics) found in semiconductor devices. The methods disclosed herein may include a cycle of hydrogen treatment steps (e.g., exposure to H₂ plasma) followed by a cleaning process (e.g., directional removal of the hydrogenated layer by low-energy Ar⁺ ions). Prior to the hydrogen treatment step, a surface modification step (e.g., exposure to oxygen plasma) may be performed to increase the surface roughness of the metal. This can be useful for maintaining the linearity of the etching process over multiple cycles. In other words, the surface modification step can allow a similar amount of metal to be etched in each cycle of the selective etching process (e.g., a metal thickness of 3 Å to 4 Å per cycle). In some embodiments, the substrate is planarized and includes an exposed Cu layer and an exposed dielectric layer in the same horizontal plane. The selective atomic layer etching method can be used to recess conductive features on the top surface of a wafer while reducing etching or damage to adjacent dielectric features, which may be beneficial for subsequent wafer bonding. In addition, selective atomic layer etching can be used to perform film surface finishing or film surface repair on the top surface of a metal (e.g., Cu).

[0022] Embodiments of this disclosure are described in the context of the accompanying drawings. An embodiment of the atomic layer etching (ALE) method for metals will be described herein using Figures 1A-1F. Experimental results of the linearity of metal etching within one cycle of an embodiment of the atomic layer etching (ALE) method for metals will be described herein using Figure 2. An application of an embodiment of the metal etching method to a hybrid bonding process flow will be described herein using Figures 3A-3D. An embodiment of a method for bonding wafers will be described herein using Figures 4 and 5. An embodiment of a plasma processing system for selective metal etching will be described herein using Figures 6, 7, 8, and 9. An embodiment of a method for etching metals will be described herein using Figures 10 and 11. An interaction between hydrogen atoms and surfaces of different roughness will be described herein using Figures 12A and 12B. An experimental result showing a decrease in the amount of metal etched per cycle with increasing additional cycles will be described herein using Figure 13. An embodiment of a method for surface smoothing will be described herein using Figures 14A-14D and Figure 15.

[0023] Figures 1A-1F illustrate methods of vapor phase atomic layer etching (ALE) of a metal (e.g., Cu) according to some embodiments. Figure 1A shows a cross-sectional view of a substrate 102 and a metal layer 104 above the substrate 102. In various embodiments, the substrate 102 is a silicon wafer with a diameter ranging from 100 mm to 500 mm, such as 150 mm, 200 mm, 300 mm, or 450 mm. In various embodiments, the substrate 102 is part of or contains a semiconductor device and may undergo multiple processing steps after conventional processes (e.g., back-end (BEOL) processes). Therefore, the substrate 102 can contain semiconductor layers useful in various microelectronics. For example, a semiconductor structure may include a substrate 102 in which various device regions are formed.

[0024] In one or more embodiments, substrate 102 may be a silicon wafer or a silicon-on-insulator (SOI) wafer. In some embodiments, substrate 102 may comprise silicon germanium, silicon carbide, gallium arsenide, gallium nitride, or other compound semiconductors. In other embodiments, substrate 102 comprises heterolayers, such as silicon germanium on silicon, gallium nitride on silicon, silicon carbon on silicon, and silicon layers on silicon or SOI substrates. In various embodiments, substrate 102 is patterned or embedded in other components of a semiconductor device. In some embodiments, substrate 102 includes conductive features (e.g., metal lines) embedded therein. The conductive features may be electrically coupled to an active device (not shown) further embedded in substrate 102.

[0025] The metal layer 104 comprises a metal such as copper (Cu). The metal layer 104 can be formed using suitable processes, such as electroplating, CVD, PVD, ALD, or combinations thereof. In various embodiments, the metal layer 104 is part of a conductive feature, such as a metal line or metal pad for a subsequent bonding process with another substrate (see below, Figures 3A-3D).

[0026] In FIG. 1B, a surface treatment 106 is performed on a metal layer 104. The surface treatment 106 includes exposing the metal layer 104 to a surface-modifying gas 108, which modifies the top surface of the metal layer 104. In some embodiments, the surface-modifying gas 108 contains oxygen and the modification of the top surface of the metal layer 104 is an oxidation process. In one example, the surface-modifying gas 108 contains plasma-excited O2 gas. In other examples, the surface-modifying gas contains one or more other reactive or oxidizing precursors, such as other oxygen-containing or oxygen-free gases capable of chemically oxidizing a metal (e.g., Cu). In other words, the surface-modifying gas system increases the oxidation state of the metal on the top surface of the metal layer 104 (e.g., from zero to +1), as can be illustrated by the particles of the surface-modifying gas 108 attached to the metal layer 104. In various embodiments where the metal layer 104 is a Cu layer and the surface-modifying gas 108 contains oxygen, the oxidation of the surface of the metal layer 104 forms a CuO x substance. The surface roughness of metal layer 104 can be increased by chemically reducing the oxide surface of metal layer 104 to an elemental metal (e.g., Cu) (see below, Figure 1C). The increase in the surface roughness of the Cu metal layer can increase the amount of metal etched by vapor phase atomic layer etching (ALE), as described below with respect to Figure 1F.

[0027] In some embodiments, the surface treatment 106 is omitted, and subsequent hydrogen treatment is performed directly on the exposed metal layer 104. This may be beneficial for increasing the roughness of the top surface of the metal layer 104.

[0028] In Figures 1C and 1D, the metal layer 104 is subjected to hydrogen treatment 110 (also referred to as a hydrogen treatment step). In some embodiments, hydrogen treatment 110 includes exposing the metal layer 104 to plasma-excited H2 gas. Figure 1C illustrates that hydrogen treatment 110 chemically reduces the oxide surface (e.g., by removing oxygen atoms). Figure 1D illustrates that hydrogen treatment 110 further forms a reaction product 114 on the metal layer 104. In some embodiments where the metal layer 104 is a Cu layer, the reaction product 114 comprises, for example, CuH2. In one example, under most high plasma density discharges and without a bias acceleration voltage, the self-bias voltage of hydrogen treatment 110 will be less than (e.g., a few volts) the sputtering threshold (87.3 eV) of Cu metal with H+ ions. In this way, physical sputtering of Cu metal from the metal layer 104 (e.g., exposure to plasma-excited H2 gas) is avoided during hydrogen treatment 110.

[0029] Next, in FIG. 1E, a cleaning process 120 is performed to remove reaction products 114 from the top surface of the metal layer 104. In some embodiments, the cleaning process 120 includes exposing the metal layer 104 to a plasma-excited inert gas system that physically expels and removes reaction products 114 (e.g., Cu-containing reaction products, such as CuH x) from the metal layer 104. The combination of the formation and removal of reaction products 114 thus etches the metal layer 104 as part of an atomic layer etching (ALE) process cycle.

[0030] As an example of cleaning process 120, the inert gas may include argon (Ar), krypton (Kr), neon (Ne), another inert gas, or a combination thereof. In some embodiments, a bias voltage is applied to substrate 102 during cleaning process 120 (e.g., when exposed to plasma-excited inert gas). In one example, the bias voltage is selected such that the kinetic energy of Ar+ ions is lower than the sputtering threshold (15.8 eV) used to remove metal (e.g., Cu) from metal layer 104. In this way, physical sputtering of metal from metal layer 104 can be largely avoided during exposure to plasma-excited Ar gas. In some examples, the bias voltage may be 30 V or less, 20 V or less, or 15 V or less.

[0031] In some embodiments, hydrogen treatment 110 (e.g., exposure to plasma-excited H2 gas) is performed first, followed by cleaning treatment 120 (e.g., exposure to plasma-excited inert gas). In some embodiments, hydrogen treatment 110 and cleaning treatment 120 may at least partially overlap in time.

[0032] In one example, the respective top surfaces of the exposed metal layer 104 and the exposed dielectric layer (e.g., SiO2) are planarized in the same horizontal plane above the substrate 102. An example of this structure is shown in FIG. 3A below, where the conductive features 210 of the metal layer and the dielectric layer 208 have coplanar top surfaces. Performing the exposure steps described above with respect to FIG. 1B-1E can provide selective etching of the metal (e.g., Cu) relative to the dielectric (e.g., SiO2), while avoiding sputtering removal of SiO2, since the sputtering threshold for removing SiO2 with Ar+ ions is approximately 50 eV. Therefore, the atomic layer etching (ALE) process shown with respect to FIG. 1B-1E can be used to selectively etch Cu metal relative to SiO2 and many other dielectrics. Other dielectrics include high-k materials such as HfO2, ZrO2, Al2O3, TiO2, etc.

[0033] Figure 1F shows the metal layer 104 after one or more etching cycles have been performed to recess the metal layer 104. The etching cycles of surface treatment 106 (e.g., exposing the metal layer 104 to a surface-modifying gas), hydrogen treatment 110 (e.g., exposing the metal layer 104 to plasma-excited H₂ gas), and cleaning treatment 120 (e.g., exposing the metal layer 104 to a plasma-excited inert gas, such as argon) can be repeated at least once to further etch the metal layer 104. The steps of the etching cycles are self-limiting, thus the atomic layer etching (ALE) process provides excellent control over the etching rate and etching uniformity. In some embodiments, when surface treatment 106 is included in the etching cycle, each etching cycle removes a thickness of 3 Å to 4 Å of metal. According to some embodiments, the step of exposing the metal layer 104 to the surface-modifying gas can be omitted from some etching cycles, such as in the last etching cycle and the etching cycle immediately preceding it. This can be advantageous for increasing the smoothness of the top surface of the metal layer 104.

[0034] Figure 2 shows experimental results of the linearity of metal etching of a metal layer 104 (e.g., a Cu metal layer) according to some embodiments in one cycle of the atomic layer etching (ALE) process described above with respect to Figures 1A-1F. The experimental results show that when the etching cycle of the ALE process described above with respect to Figures 1A-1F is performed multiple times and the surface treatment 106 step (e.g., exposing the metal layer 104 to a surface-modifying gas, such as plasma-excited O2 gas) is omitted, the amount of Cu metal etched in each etching cycle decreases with the increase of the number of etching cycles. This can be attributed to the smoothing effect of increasing the smoothness of the Cu metal layer 104 after each etching cycle. The smoothness of the surface of the Cu metal layer 104 has the effect of reducing the amount of Cu-containing reaction products (e.g., CuHx) formed on the Cu metal layer 104 during the hydrogen treatment 110 (e.g., exposing the metal layer 104 to plasma-excited H2 gas). This may further lead to a reduction in the etching of Cu metal in each etching cycle. The Cu etch depth per etch cycle can be continuously reduced from approximately 3 Å per etch cycle to approximately 0.4 Å per etch cycle, as shown by the trace 0A Ar sputtering in Figure 2.

[0035] In some embodiments, this smoothing effect and the resulting reduction in Cu etching amount in each etching cycle can be addressed by including a surface treatment 106 (e.g., exposing the metal layer 104 to plasma-excited O2 gas) in one or more etching cycles. Oxidizing the surface of the Cu metal layer 104 to form CuO x material in the surface treatment 106 (also known as a surface modification step) and then chemically reducing the oxidized surface of the Cu metal layer to elemental Cu during hydrogen treatment 110 (e.g., exposing the metal layer 104 to plasma-excited H2 gas) is considered to increase the surface roughness of the Cu metal layer 104. The increase in the surface roughness of the Cu metal layer 104 increases the Cu etching amount in each cycle and restores the linearity of the Cu etching amount in each cycle. The surface modification step provides an alternative to physically sputtering the Cu metal layer 104 with high-energy Ar+ ions to at least partially restore the initial surface roughness of the Cu metal layer. As shown in the O2 treatment traces in Figure 2, the etch linearity is restored when a surface modification step exposing the substrate to plasma-excited O2 gas is introduced at the beginning of each etch cycle. However, due to variations in process conditions, the amount of etch per cycle is smaller. Furthermore, the plasma-exposed O2 gas system is processed using process conditions that do not result in physical sputtering of Cu metal.

[0036] For comparison, Figure 2 further shows Cu etching using atomic layer etching, where a Cu sputtering step is added at the beginning of each etching cycle as an alternative to a surface modification step (e.g., exposing metal layer 104 to plasma-excited O2 gas). The Cu sputtering step removes approximately 2 Å (trace 2 Å Ar sputtering) and 4 Å (trace 4 Å Ar sputtering) of Cu metal. The Cu sputtering step is performed by applying a bias of approximately 80 eV to the substrate (well above the sputtering threshold of Ar+ for Cu metal) and exposing the Cu metal layer to plasma-excited Ar gas. The results show that the Cu sputtering step increases the amount of Cu metal etched subsequently during exposure to plasma-excited H2 gas, followed by exposure to plasma-excited inert gas. This is attributed to the increased surface roughness of the Cu metal layer by the physical sputtering of high-energy Ar+ ions.

[0037] The results in Figure 2 show that oxidation of the Cu layer by a surface modification step (e.g., exposing the metal layer 104 to plasma-excited O2 gas) can lead to an increase in surface roughness similar to that of the Cu sputtering step. Furthermore, the Cu etching amount and etching linearity can be increased by a surface modification step (e.g., exposing the metal layer 104 to plasma-excited O2 gas), where the surface modification step does not result in physical sputtering of the Cu metal. Therefore, undesirable Cu sputtering and etching of dielectric portions adjacent to the Cu metal layer can be reduced or avoided in the Cu etching process.

[0038] In some embodiments, each ALE cycle includes a hydrogen treatment step in H2 plasma, followed by a cleaning step (also known as an Ar plasma step) using Ar+ bombardment. No accelerating voltage is applied to the substrate during the H2 plasma exposure. In various embodiments, the ion bombardment energy is 100 eV or less. Precise ion energy control during the subsequent Ar plasma step can be achieved by applying a DC pulse waveform to the substrate at a given voltage (e.g., with an on time of 1 µs and an off time of 2 µs per cycle). In some embodiments, the hydrogen treatment step is performed with a gas composition of 40% H2 and 60% Ar by volume, a gas pressure of 100 mTorr, an exposure time of 60 seconds, and an ICP plasma power of 200 W. In some embodiments, the cleaning step (e.g., exposure to plasma-excited Ar gas) is performed using 50 mTorr of Ar gas, a bias of 20 V, using DC, and an exposure time of 120 seconds at 33% of the duty cycle (e.g., 1 µs on and 2 µs off per cycle). In various embodiments, the surface modification step (e.g., exposing the Cu metal layer to plasma-excited O2 gas) is performed prior to the hydrogen treatment and cleaning steps in one or more ALE cycles. In some embodiments, the surface modification step is performed with a gas composition of 50% O2 and 50% Ar by volume, a gas pressure of 50 mTorr, and an exposure time of 30 seconds at a bias of 0 V. One example of an experimental physical sputtering step used for a surface modification concept demonstration is performed using 100% Ar at 50 mTorr and a pulsed DC voltage of 80 V. By changing the sputtering time from 0 seconds to 60 seconds, it is possible to physically sputter the surface material at 0 Å, 2 Å, or 4 Å, as shown by the corresponding traces of 0 Å Ar sputtering, 2 Å Ar sputtering, and 4 Å Ar sputtering in Figure 2.

[0039] Figures 3A-3D illustrate how the metal etching method described above with respect to Figures 1A-2 can be applied to an exemplary three-dimensional integration (3DI) hybrid bonding process flow. The wafer bonding process includes bonding a top wafer 200 to a bottom wafer 202. Prior to the wafer bonding process, it is advantageous to precisely remove 2 nm to 5 nm of metal (e.g., Cu) from the top surface of the top wafer 200 to the bottom wafer 202, wherein the metal removal process is selective for the dielectric (e.g., SiO2) on the top surfaces of both the top wafer 200 and the bottom wafer 202. This reduces damage to the dielectric surface, reduces the increase in dielectric surface roughness, and reduces dielectric fillets to reduce or avoid voids after the wafer bonding process. Furthermore, the metal removal process benefits from non-corrosive chemicals and advantageously reduces or eliminates contamination of the process chamber by volatile reaction products (e.g., Cu-containing reaction products). The metal etching method described above with respect to Figure 1A-2 provides the aforementioned advantages.

[0040] Figure 3A shows a cross-sectional view of the top wafer 200 and the bottom wafer 202 to be joined together. In some embodiments, each of the top wafer 200 and the bottom wafer 202 includes a respective substrate 102 and a CMOS back-end process (BEOL) layer 206 above the substrate 102. The BEOL layer 206 may include active devices (e.g., transistors, such as CMOS devices) coupled via conductive features in the BEOL layer 206.

[0041] Each of the top wafer 200 and the bottom wafer 202 further includes a conductive feature 210 above the BEOL layer 206. In various embodiments, the conductive feature is a metal line, a metal pad, etc., and includes copper (Cu). The conductive feature 210 is separated by a dielectric layer 208 above the BEOL layer 206. The dielectric layer 208 contains a suitable dielectric material, such as SiO2. The conductive feature 210 can be formed in the dielectric layer 208 by an acceptable process, such as a damascene process.

[0042] Figure 3B shows the top wafer 200 and bottom wafer 202 after a selective etching process in which the conductive features 210 are recessed below the corresponding top surface of the dielectric layer 208. The selective metal etching method described above with respect to Figures 1A-1F can be used for this step.

[0043] In FIG3C, an initial dielectric-dielectric bonding is performed between the respective dielectric layers 208 of the top wafer 200 and the bottom wafer 202. In some embodiments, the dielectric-dielectric bonding is a SiO2-to-SiO2 bonding.

[0044] In FIG. 3D, the individual conductive features 210 of the top wafer 200 and the bottom wafer 202 are bonded to complete a hybrid bonding between the top wafer 200 and the bottom wafer 202. Metal reflow can be performed to perform heat treatment to fill the gap between the top wafer 200 and the bottom wafer 202. In some embodiments, the metal of the conductive features 210 is copper (Cu), and the heat treatment causes Cu reflow, which creates Cu-to-Cu bonding between the top wafer 200 and the bottom wafer 202.

[0045] In some embodiments, the selective method of metal etching described above with respect to Figures 1A-1F is performed in a stand-alone etching platform, and the top wafer 200 and the bottom wafer 202 are subsequently transferred to a separate bonding system to perform heat treatment and any surface treatment performed prior to the heat treatment, as described below with respect to Figure 4. An exemplary bonding system for performing the bonding process of Figures 3C-3D is described in U.S. Patent No. 11,424,142, which is incorporated herein by reference in its entirety. In other embodiments, the etching platform for performing the selective method of metal etching described above with respect to Figures 1A-1F and the bonding system for bonding the top wafer 200 and the bottom wafer 202 are integrated into a single system, as described below with respect to Figure 5.

[0046] Figure 4 illustrates a process flow diagram illustrating a method 400 for bonding wafers using an etching platform and a separate bonding device according to some embodiments. In step 402, an atomic layer etching (ALE) process (e.g., the selective metal etching method described above with respect to Figures 1A-1F) is performed on the etching platform described above with respect to Figures 3A-3B on the corresponding metal feature portions 210 of the first wafer 200 and the second wafer 202. In step 404, the first wafer 200 and the second wafer 202 are moved to a bonding device separate from the etching platform. In some embodiments, the first wafer 200 and the second wafer 202 may be transferred from the etching platform to the bonding device after an appropriate queuing time.

[0047] In step 406, the surfaces of the first wafer 200 and the second wafer 202 are modified to prepare them for a bonding process. The surface modification of the first wafer 200 and the second wafer 202, and the preparation for the bonding process, may include the following steps: cleaning the first wafer 200 and the second wafer 202; inspecting the first wafer 200 and the second wafer 202 for unwanted deposits; modifying the surfaces of the first wafer 200 and the second wafer 202 (e.g., with oxygen plasma); hydrophilizing the surfaces of the first wafer 200 and the second wafer 202 (e.g., by flowing pure water over the first wafer 200 and the second wafer 202 while simultaneously rotating them); and positioning the first wafer 200 on top of the second wafer 202 in a bonding apparatus. However, any suitable modification and preparation steps may be used.

[0048] In step 408, the first wafer 200 and the second wafer 202 are joined in a bonding apparatus by a suitable bonding process. For example, the first wafer 200 and the second wafer 202 can be positioned together by their respective chucks and joined together by a bonding wave generated by a pressure pin that presses the first wafer 200 and the second wafer 202 together at the center point. However, any suitable bonding process for wafers can be used.

[0049] Figure 5 shows a process flow diagram illustrating a method 500 for bonding wafers using an integrated etching platform including bonding equipment, according to some embodiments. In step 502, an atomic layer etching (ALE) process (e.g., the selective metal etching method described above with respect to Figures 1A-1F) is performed on corresponding metal feature portions 210 of the first wafer 200 and the second wafer 202, as described above with respect to Figures 3A-3B. This process is performed in an integrated etching platform having process chambers configured to perform etching and bonding processes.

[0050] In step 504, the surfaces of the first wafer 200 and the second wafer 202 are modified to prepare them for a bonding process. The surface modification and bonding preparation performed on the first wafer 200 and the second wafer 202 can be performed using a method similar to that described above with respect to step 406 of FIG. 4. In step 506, the first wafer 200 and the second wafer 202 are bonded in an integrated etching platform using a suitable bonding process. This bonding process can be performed using a method similar to that described above with respect to step 408 of FIG. 4.

[0051] Figures 6-9 show cross-sectional views illustrating plasma processing systems according to various embodiments for performing the metal etching selective methods described above with respect to Figures 1A-1F.

[0052] Figure 6 shows a plasma processing system 4 according to some embodiments. The plasma processing system 4 includes a chamber 10, a substrate support 20 on which a substrate 25 to be processed is fixed, a gas injection system 40, and a vacuum pump system 50. The chamber 10 is configured to facilitate the generation of plasma in a processing area 45 adjacent to the surface of the substrate 25, wherein the plasma is formed by collisions between heated electrons and ionizable gases. Ionizable gases or gas mixtures are introduced via the gas injection system 40 and the process pressure is regulated. For example, a gate valve (not shown) is used to throttle the vacuum pump system 50. The plasma is used to generate plasma-excited gaseous material for removing material from the exposed surface of the substrate 25.

[0053] The substrate 25 is transported into and out of the chamber 10 via a slot valve (not shown) and a chamber feeder device (not shown) through a robotic substrate transport system. The substrate 25 is received by a substrate lifting pin (not shown) housed in the substrate support 20 and is mechanically moved by a device installed therein. Once the substrate 25 is received from the substrate transport system, the substrate 25 is lowered onto the upper surface of the substrate support 20.

[0054] In other embodiments, the substrate 25 is secured to the substrate support 20 by electrostatic clamps (not shown). Furthermore, the substrate support 20 may also include a cooling system comprising a recirculating coolant stream that receives heat from the substrate support 20 and transfers the heat to a heat exchanger system (not shown) when operating in cooling mode. When operating in heating mode, the cooling system may also transfer heat from the heat exchanger system. Gas may be supplied to the back side of the substrate 25 to increase the air gap thermal conductivity between the substrate 25 and the substrate support 20. Such a system is used when it is necessary to control the temperature of the substrate at elevated or lower temperatures. For example, temperature control of the substrate at temperatures exceeding a steady-state temperature is useful, where the steady-state temperature is achieved by balancing the heat flux transferred from the plasma to the substrate 25 and the heat flux removed from the substrate 25 by conduction to the substrate support 20. In other embodiments, heating elements, such as resistance heating elements or thermoelectric heaters / coolers, are included in the substrate support 20.

[0055] Still referring to FIG. 6, the substrate holder 20 further serves as an electrode through which RF power is coupled to the plasma in the processing region 45. For example, the substrate holder 20 can be electrically biased at an RF voltage via the transmission of RF power supplied to the substrate holder 20 from the RF generator 30 through an impedance matching network 32. The RF bias is used to form and sustain the plasma by exciting electrons. In this configuration, the system operates as a reactive ion etching (RIE) reactor, where the chamber and upper gas injection electrodes serve as ground surfaces. Typical frequencies for RF bias can range from 1 MHz to 100 MHz, for example, 13.56 MHz. In other embodiments, the substrate holder 20 can be electrically biased using a pulsed DC voltage. For example, a pulsed DC voltage can be applied at frequencies ranging from 100 Hz to 10 MHz, with a pulse duty cycle ranging from 1% to 99%, which corresponds to continuous wave (CW) RF power. Additionally, if desired, the pulsed DC voltage can be modulated at a lower frequency (e.g., less than 100 Hz) to perform a pulse train or pulse wave.

[0056] In one example, RF power is applied to the substrate support electrodes at multiple frequencies. Furthermore, the impedance matching network 32 can increase the transmission of RF power to the plasma in the processing chamber 10 by reducing reflected power. The impedance matching network 32 can have any suitable matching network layout (e.g., L-type, π-type, T-type, etc.) and / or automatic control method known in the art.

[0057] Process gas 42 (also known as etching gas) is introduced into processing area 45 via gas injection system 40. In some embodiments, gas injection system 40 includes a spray head, wherein process gas 42 is supplied from gas delivery system (not shown) through gas injection chamber (not shown), a series of baffles (not shown) and porous spray head gas injection plate (not shown) to processing area 45.

[0058] The vacuum pump system 50 may include a turbomolecular vacuum pump (TMP) with a pumping speed of up to 5000 liters per second (or higher) and a gate valve for throttling the chamber pressure. In some plasma processing apparatuses used for dry plasma etching processes, TMPs of 1000 to 3000 liters per second may be used. TMPs can be used for low-pressure processes, for example, at pressures less than 50 mTorr. At higher pressures, the pumping speed of the TMP pump may decrease drastically. For high-pressure processes (e.g., greater than 100 mTorr), mechanical booster pumps and dry roughing pumps may be used.

[0059] Computer 55 includes a microprocessor, memory, and digital I / O ports, capable of generating control voltages sufficient to transmit and activate inputs to plasma processing system 4 and monitor outputs from plasma processing system 4. Computer 55 is further coupled to and exchanges information with RF generator 30, impedance matching network 32, gas injection system 40, and vacuum pump system 50. A program stored in the memory of computer 55 is used to initiate inputs to the aforementioned components of plasma processing system 4 according to a stored process formula. In various embodiments, the program includes instructions to expose a metal layer on a substrate supported by substrate support 20 to an oxidizing gas, to expose the metal layer to plasma-excited H2 gas, and to expose the metal layer to plasma-excited Ar gas, as described above with respect to Figures 1A-1F.

[0060] Figure 7 shows a plasma treatment system 5 according to some embodiments. In addition to the various components of the plasma treatment system 4 described above with reference to Figure 6, the plasma treatment system 5 also includes a mechanical or electro-rotating DC magnetic field system 60 to potentially increase plasma density and / or improve plasma treatment uniformity. Furthermore, a computer 55 is coupled to the rotating magnetic field system 60 and is configured to adjust the rotation speed and magnetic field strength.

[0061] Figure 8 shows a plasma processing system 6 according to some embodiments. In addition to the various components of the plasma processing system 4 described above with reference to Figure 6, the plasma processing system 6 also includes an upper electrode 70, to which RF power is coupled from an RF generator 72 via an impedance matching network 74. The upper electrode can be powered at frequencies ranging from 10 MHz to 200 MHz (e.g., 60 MHz). The lower electrode can be powered at frequencies ranging from 0.1 MHz to 60 MHz (e.g., 40 MHz). Furthermore, a computer 55 is coupled to the RF generator 72 and the impedance matching network 74 and is configured to control the RF power supplied to the upper electrode 70.

[0062] Figure 9 shows a plasma processing system 7 according to some embodiments. The plasma processing system 7 is an inductively coupled plasma (ICP) system, which, in addition to the components of the plasma processing system 4 described with reference to Figure 6 above, further includes an induction coil 80. RF power is coupled to the induction coil 80 via an impedance matching network 84 through an RF generator 82. The RF power is inductively coupled from the induction coil 80 to the plasma processing region 45 through a dielectric window (not shown). The RF power can be supplied to the induction coil 80 at a frequency in the range of 400 kHz to 100 MHz (e.g., 13.56 MHz). Furthermore, a slit Faraday shield (not shown) is used to reduce capacitive coupling between the induction coil 80 and the plasma in the plasma processing region 45. In addition, a computer 55 is coupled to the RF generator 82 and the impedance matching network 84 and is configured to control the power applied to the induction coil 80. RF bias power can be applied to the substrate support 20 at a frequency in the range of 1 MHz to 100 MHz. In other embodiments, the substrate holder 20 can be electrically biased using a pulsed DC voltage. For example, a pulsed DC voltage can be applied at a frequency in the range of 100 Hz to 10 MHz, with the pulse duty cycle in the range of 1% to 99%.

[0063] In other embodiments, a frequency in the range of 300 MHz to 9 GHz (e.g., 2.54 GHz) is used to form plasma using electron cyclotron resonance (ECR). In still other embodiments, plasma is formed by the emission of helical waves. In still other embodiments, plasma is formed by propagating surface waves.

[0064] FIG10 illustrates a process flow diagram of a method 1000 for etching metal according to some embodiments. In step 1002, surface modification 106 is performed on the exposed surface of the metal layer 104 above the substrate 102, as described above with respect to FIG1B. Surface modification increases the surface roughness of the metal layer 104.

[0065] In step 1004, the metal layer 104 is subjected to hydrogen treatment 110, as described above with respect to Figures 1C-1D. Hydrogen treatment 110 forms a layer of reaction product 114 on the metal layer 104. In step 1006, a cleaning treatment 120 is performed on the metal layer 104, as described above with respect to Figure 1E. Cleaning treatment 120 removes the layer of reaction product 114. In various embodiments, one cycle of the etching process includes steps 1002, 1004, and 1006, and the etching process is performed for at least two cycles. In some embodiments, the surface modification step 1002 is omitted in one or more etching cycles, which may be beneficial for increasing the smoothness of the top surface of the metal layer 104.

[0066] FIG11 illustrates a process flow diagram of a method 1100 for etching metal according to some embodiments. In step 1102, a substrate 102 having a Cu metal layer 104 above a substrate 102 is provided, as described above with respect to FIG1A.

[0067] Next, an etching process is performed, wherein the first cycle of the etching process includes steps 1104 to 1108. In step 1104, the Cu metal layer 104 is exposed to a surface-modifying gas (e.g., plasma-excited O2 plasma) in surface treatment 106, as described above with respect to FIG. 1B. The surface-modifying gas oxidizes the surface of the Cu metal layer 104. In step 1106, the Cu metal layer 104 is exposed to plasma-excited H2 gas 112, as described above with respect to FIG. 1C-1D. The plasma-excited H2 gas 112 chemically reduces the oxidized surface of the Cu metal layer 104 and forms a Cu-containing reaction product layer 114 on the Cu metal layer 104.

[0068] In step 1108, the Cu metal layer 104 is exposed to plasma-excited Ar gas in cleaning process 120, as described above with respect to FIG. 1E. The plasma-excited Ar gas removes the Cu-containing reaction products 114 covering the Cu metal layer 104. In some embodiments, the kinetic energy of the plasma-excited Ar gas is at or below the sputtering threshold for removing Cu metal from the Cu metal layer 104. Subsequently, additional cycles of the etching process may be performed, each additional cycle comprising steps 1104 to 1108. In some embodiments, step 1104, which involves exposing the Cu metal layer 104 to the surface-modifying gas, is omitted in one or more cycles of the etching process, which may be beneficial for increasing the smoothness of the top surface of the Cu metal layer 104.

[0069] Selective atomic layer etching (ALE) methods are useful for performing film surface finishing or repair on the top surface of a metal (e.g., Cu). Metal films, such as Cu films, may have native oxides or other generally rough surfaces on their exposed surfaces. Film surface finishing or smoothing can be beneficial, for example, in improving the bonding of Cu metal pads during hybrid bonding of wafers. The methods disclosed herein allow for increasing the smoothness of metal surfaces by selectively removing several atomic layers in a cyclic manner (e.g., after several atomic layer etching (ALE) cycles). In some embodiments, a highly selective, self-limiting, cyclic two-step process is used to remove native oxides and repair the surface of the metal surface (e.g., Cu surface). This surface treatment allows for the selective nanoscale repair of small metal (e.g., Cu) pads and regions on patterned structures.

[0070] According to some embodiments, Figures 12A and 12B illustrate the interaction of hydrogen atoms with surfaces of different roughness. Figure 12A shows a cross-sectional view of a rough metal surface 304, such as a close-up view of the top surface of metal layer 104 (see above, Figures 1A-1B). The metal surface 304 is rough and has space between the top atoms of its metal lattice. In different examples, the roughness of the metal surface 304 is the result of surface modification 106 (see above, Figure 1B) or Ar+ sputtering. Due to the roughness of the metal surface 304, hydrogen atoms (e.g., of plasma-excited H2 gas 112) may more easily enter the space of the metal surface 304 and react with and / or chemically adsorb the atoms of the metal surface 304. This may increase the amount of material removed in subsequent cycles of the ALE process (see above, Figures 1C-1E).

[0071] Figure 12B shows a cross-sectional view of the smooth metal surface 306, such as a close-up view of the top surface of the metal layer 104 before surface modification 106 or after several cycles of the ALE process (see above, Figure 1F). The metal surface 306 is smooth and has a more uniform atomic distribution. Due to the smoothness of the metal surface 306, hydrogen atoms (e.g., hydrogen atoms from plasma-excited H2 gas 112) may have more difficulty reacting with the metal atoms of the metal surface 306. This may reduce the amount of material removed in subsequent cycles of the ALE process.

[0072] Figure 13 shows the experimental results of the amount of material etched from the metal surface per cycle, also known as the etch amount per cycle (EA), which decreases with each additional cycle of the atomic layer etching (ALE) process. The experimental results shown in Figure 13 are a detailed view of the cumulative etch amount trace 0A Ar sputtering shown in Figure 2 above. Each cycle of the ALE process includes a hydrogen treatment step (also known as modification) and a cleaning step (also known as Ar+ bombardment). In the experimental results shown, the hydrogen treatment step was performed at a gas pressure of 100 mTorr, an exposure time of 60 seconds, and a plasma power of 200 W without bias, while the cleaning step was performed for ten cycles of the ALE process under Ar gas conditions of 50 mTorr, a plasma power of 200 W, and a bias of 20 V. As shown in Figure 13, the EA per cycle decreases with each additional cycle. This may be because the decrease in surface roughness after each cycle may inhibit the reaction and chemisorption of hydrogen atoms on the metal surface, as described above regarding Figures 12A and 12B. Therefore, repeatable cycles of the ALE process, which includes hydrogen treatment and cleaning steps, can be used to perform film surface finishing or film surface repair on the top surface of a metal (e.g., Cu).

[0073] Figures 14A to 14D illustrate methods for smoothing a metal surface according to some embodiments. Figure 14A shows a cross-sectional view of a substrate 102 and a metal layer 104 above the substrate 102. A rough or oxidized surface 320 is present above the metal layer 104. In various embodiments, the rough or oxidized surface 320 comprises a natural oxide and is a substantially rough surface of a newly formed copper film, or has been oxidized by surface modification 106 (see above, Figure 1B). In some embodiments, a mask 316 is formed over the metal layer 104 to expose a portion of the rough or oxidized surface 320. The mask 316 may be, for example, a photoresist patterned by a suitable photolithography process. However, any suitable mask 316 may be used.

[0074] In FIG. 14B, hydrogen treatment 110 is performed on the rough or oxidized surface 320. In some embodiments, hydrogen treatment 110 includes exposing the metal layer 104 to plasma-excited H2 gas. Hydrogen treatment 110 can be performed using a similar method as described above with respect to FIG. 1B-1C. FIG. 14B shows that hydrogen treatment 110 activates the rough or oxidized surface 320 by further forming a layer of reaction product 114 on the rough or oxidized surface 320.

[0075] In FIG. 14C, a cleaning process 120 is performed to remove reaction products 114 from the top surface of the roughened or oxidized surface 320. In some embodiments, the cleaning process 120 includes exposing the roughened or oxidized surface 320 to a plasma-excited inert gas, which physically expels and removes the reaction products 114 (e.g., Cu-containing reaction products, such as CuHx) from the roughened or oxidized surface 320. The cleaning process 120 can be performed using a similar method as described above with respect to FIG. 1E, for example, bombardment with low-energy Ar+ ions. The combination of the formation and removal of reaction products 114 thus removes atomic layers from the roughened or oxidized surface 320 as part of one cycle of an atomic layer etching (ALE) process.

[0076] Figure 14D illustrates a cross-sectional view of the structure after multiple cycles of the ALE process have removed the rough or oxidized surface 320. After removing several angstrom-level atomic layers of the rough or oxidized surface 320, the top surface of the remaining metal layer 104 has increased smoothness. Multiple cycles of the ALE process allow for nanoscale selective repair of small metal (e.g., Cu) pads and regions on patterned structures. In some embodiments where a mask 316 is present, the mask 316 is removed by a suitable process (e.g., ashing in oxygen plasma) after the ALE process is completed.

[0077] FIG15 illustrates a process flow diagram of a surface smoothing method 1200 according to some embodiments. In step 1202, a substrate 102 having a metal layer 104 over a substrate 102 is provided, wherein the metal layer 104 has a rough or oxidized surface 320, as described above with respect to FIG14A. In some embodiments, the rough or oxidized surface 320 is the product of surface modification 106, which is performed as a step in method 1000 or 1200 of etching metal, as described above with respect to FIGS. 10 and 11, respectively.

[0078] Next, an etching process is performed, wherein the first cycle of the etching process includes steps 1204 and 1206. In step 1204, the roughened or oxidized surface 320 is exposed to plasma-excited H2 gas 112, as described above with respect to FIG. 14B. The plasma-excited H2 gas 112 chemically reduces the roughened or oxidized surface 320 and forms a layer of reaction product 114 on the roughened or oxidized surface 320.

[0079] In step 1206, the roughened or oxidized surface 320 is exposed to plasma-excited Ar gas in cleaning process 120, as described above with respect to FIG. 14C. The plasma-excited Ar gas removes the reaction products 114 covering the roughened or oxidized surface 320. In some embodiments, the kinetic energy of the plasma-excited Ar gas is at or below the sputtering threshold for removing metal from the metal layer 104. Next, additional cycles of the etching process may be performed, each additional cycle comprising steps 1204 and 1206, until the roughened or oxidized surface 320 or the exposed surface of the metal layer 104 achieves the desired smoothness.

[0080] Exemplary embodiments of the disclosure are summarized herein. Other embodiments may also be understood from the entire specification and the scope of the claims filed herein.

[0081] Example 1. A metal etching method comprising: performing an etching process for at least two cycles, wherein each cycle comprises: performing surface modification on an exposed surface of a metal layer above a substrate; performing hydrogen treatment on the metal layer, wherein the hydrogen treatment forms a layer of reaction products on the metal layer; and performing a cleaning process on the metal layer, wherein the cleaning process removes the layer of reaction products.

[0082] Example 2. The method of Example 1, wherein the surface modification system comprises oxidizing the exposed surface of the metal layer.

[0083] Example 3. The method of one of Example 1 or 2, wherein the hydrogen treatment system includes exposing the metal layer to plasma-excited H2 gas.

[0084] Example 4. The method of one of Examples 1 to 3, wherein the cleaning process includes exposing the metal layer to a plasma-excited inert gas.

[0085] Example 5. As in one of Examples 1 to 4, wherein the first cycle of the etching process removes a thickness of 3 Å to 4 Å of the metal layer.

[0086] Example 6. A method for smoothing a metal surface, the method comprising: providing a substrate, a metal layer system being disposed above the substrate, the metal layer system having a rough or oxidized surface; and performing one or more cycles of a smoothing process, wherein each cycle comprises: exposing the rough or oxidized surface to a plasma-excited H2 gas, wherein the plasma-excited H2 gas chemically reduces the oxidized surface of the metal layer and forms a layer of reaction products on the metal layer; and exposing the rough or oxidized surface to a plasma-excited Ar gas, wherein the plasma-excited Ar gas removes the layer of reaction products covering the metal layer, wherein the kinetic energy of the plasma-excited Ar gas is at or below a sputtering threshold for removing metal from the metal layer.

[0087] Example 7. The method of Example 6 further includes: forming a mask over the metal layer, the mask exposing the rough or oxidized surface; and removing the mask after the smoothing process of one or more cycles has been completed.

[0088] Example 8. The method of one of Examples 6 or 7 further includes performing an etching process before performing the smoothing process, wherein one cycle of the etching process includes: exposing the metal layer to an oxidizing gas; exposing the metal layer to the plasma-excited H2 gas; and exposing the metal layer to the plasma-excited Ar gas.

[0089] Example 9. As in any one of Examples 6 to 8, the substrate is planarized and includes the metal layer and dielectric layer on the same horizontal plane.

[0090] Example 10. The method of Example 9, wherein the metal layer system is selectively etched relative to the dielectric layer.

[0091] Example 11. The method of Example 10, wherein the dielectric layer comprises SiO2 or a high-k material.

[0092] Example 12. The method of one of Examples 6 to 11, wherein the smoothing process uses an inductively coupled plasma (ICP) system.

[0093] Example 13. As in any one of Examples 6 to 11, the kinetic energy of the plasma-excited Ar gas is controlled by applying an RF or pulsed DC bias to the substrate.

[0094] Example 14. As in one of Examples 6 to 13, wherein the metal layer comprises copper.

[0095] Example 15. A plasma processing system includes: a chamber including a substrate support configured to support a substrate; a gas injection system coupled to the chamber; a vacuum pump system coupled to the chamber; and a computer coupled to the gas injection system and the vacuum pump system, wherein the computer is configured to execute a program including instructions to perform the following: expose a metal layer on the substrate to an oxidizing gas; expose the metal layer to plasma-excited H2 gas; and expose the metal layer to plasma-excited inert gas.

[0096] Example 16. The plasma processing system of Example 15 further includes a first RF generator coupled to the substrate support, wherein the substrate support serves as an electrode and the RF power is coupled to the plasma in the chamber through the electrode.

[0097] Example 17. The plasma processing system of Example 16 further includes a rotating DC magnetic field system surrounding the chamber.

[0098] Example 18. The plasma processing system of Example 16 further includes: an upper plate electrode located on the substrate support; and a second RF generator coupled to the upper plate electrode.

[0099] Example 19. The plasma processing system of Example 16 further includes: an induction coil surrounding the chamber; and a second RF generator coupled to the induction coil.

[0100] Example 20. A plasma processing system as in Example 19, wherein RF power in the frequency range of 400 kHz to 100 MHz is inductively coupled from the induction coil to the plasma processing region within the chamber.

[0101] Example 21. A method of etching metal, the method comprising: providing a substrate, wherein a Cu metal layer is disposed on the substrate; performing a first cycle of an etching process, the first cycle comprising: exposing the Cu metal layer to a surface-modifying gas, wherein the surface-modifying gas oxidizes the surface of the Cu metal layer; exposing the Cu metal layer to a plasma-excited H2 gas, wherein the plasma-excited H2 gas chemically reduces the oxidized surface of the Cu metal layer and forms a Cu-containing reaction product layer on the Cu metal layer; and exposing the Cu metal layer to a plasma-excited Ar gas, wherein the plasma-excited Ar gas removes the Cu-containing reaction product layer covering the Cu metal layer, wherein the kinetic energy of the plasma-excited Ar gas is at or below a sputtering threshold for removing Cu metal from the Cu metal layer.

[0102] Example 22. The method of Example 21 further includes a second cycle of performing an etching process, wherein the second cycle of the etching process includes the same steps as the first cycle.

[0103] Example 23. The method of one of Examples 21 or 22, wherein the surface-modifying gas comprises plasma-excited O2 gas.

[0104] Example 24. As in Example 23, the kinetic energy of the plasma-excited O2 gas is at or below the sputtering threshold for removing Cu metal from the Cu metal layer.

[0105] Example 25. The method of one of Examples 21 to 24, wherein the substrate system is planarized and includes a Cu metal layer and a dielectric layer in the same horizontal plane.

[0106] Example 26. The method of Example 25, wherein the Cu metal layer is selectively etched relative to the dielectric layer.

[0107] Example 27. The method of one of Examples 25 or 26, wherein the dielectric layer comprises SiO2 or a high-k material.

[0108] Example 28. The method of one of Examples 21 to 27, wherein the first cycle of the etching process utilizes an inductively coupled plasma (ICP) system.

[0109] Example 29. The method of one of Examples 21 to 27, wherein the kinetic energy of the plasma-excited Ar gas is controlled by applying an RF or pulsed DC bias voltage to the substrate.

[0110] Although the present invention has been described with reference to illustrative embodiments, this description is not intended to be limiting. Various modifications and combinations of exemplary embodiments, as well as other embodiments of the invention, will be apparent to those skilled in the art upon reference to the description. Therefore, the appended claims are intended to cover any such modifications or embodiments. [Simplified Explanation of the Diagram]

[0008] To more fully understand the present invention and its advantages, reference is now made to the accompanying drawings and the following description, wherein:

[0009] Figures 1A-1F illustrate atomic layer etching (ALE) methods for metals according to some embodiments;

[0010] Figure 2 shows experimental results of the linearity of metal etching according to some embodiments;

[0011] Figures 3A-3D show the application of metal etching methods according to some embodiments in a hybrid bonding process flow;

[0012] Figures 4 and 5 show process flow diagrams of methods for bonding wafers according to some embodiments;

[0013] Figures 6-9 illustrate a plasma processing system for selective metal etching according to some embodiments;

[0014] Figures 10 and 11 illustrate process flow diagrams of a method for etching metal according to some embodiments;

[0015] Figures 12A and 12B illustrate the interaction of hydrogen atoms with surfaces of different smoothness according to some embodiments;

[0016] Figure 13 illustrates experimental results showing that, according to some embodiments, the amount of metal etched per cycle decreases with the increase of additional cycles;

[0017] Figures 14A-14D illustrate methods for smoothing surfaces according to some embodiments; and

[0018] Figure 15 illustrates a process flow diagram of a method for smoothing a surface according to some embodiments.

[0019] Unless otherwise stated, corresponding numbers and symbols in the various figures generally refer to corresponding parts. The figures are drawn to clearly illustrate the relevant aspects of the embodiments and are not necessarily drawn to scale. Feature edges drawn in the figures do not necessarily indicate the termination of feature ranges.

Claims

1. A metal etching method, the method comprising: performing an etching process for at least two cycles, wherein each cycle comprises: performing a surface modification on an exposed surface of a metal layer above a substrate, the substrate being planarized and comprising the metal layer and a dielectric layer on a horizontal plane; performing a hydrogen treatment on the metal layer, wherein the hydrogen treatment forms a layer of reaction products on the metal layer; and performing a cleaning treatment on the metal layer, wherein the cleaning treatment removes the layer of reaction products.

2. The metal etching method of claim 1, wherein the surface modification comprises oxidizing the exposed surface of the metal layer.

3. The metal etching method of claim 1, wherein the hydrogen treatment comprises exposing the metal layer to a plasma-excited H2 gas.

4. The metal etching method of claim 1, wherein the cleaning process includes exposing the metal layer to a plasma-excited inert gas.

5. The metal etching method of claim 1, wherein a first cycle of the etching process removes a thickness of 3 Å to 4 Å from the metal layer.

6. A method for smoothing a surface, the method comprising: providing a substrate, the substrate being planarized and comprising a metal layer and a dielectric layer on a horizontal plane, the metal layer having a rough or oxidized surface; and performing one or more cycles of a smoothing process, wherein each cycle comprises: exposing the rough or oxidized surface to a plasma-excited H2 gas, wherein the plasma-excited H2 gas chemically reduces the oxidized surface of the metal layer and forms a layer of reaction products on the metal layer; and exposing the rough or oxidized surface to a plasma-excited Ar gas, wherein the plasma-excited Ar gas removes the layer of reaction products covering the metal layer, wherein the kinetic energy of the plasma-excited Ar gas is at or below a sputtering threshold for removing metal from the metal layer.

7. The method for smoothing a surface as claimed in claim 6, further comprising: forming a mask over the metal layer, the mask exposing the rough or oxidized surface; and removing the mask after one or more cycles of the smoothing process have been completed.

8. The method for smoothing a surface as claimed in claim 6, further comprising performing an etching process prior to performing the smoothing process, wherein a cycle of the etching process includes: exposing the metal layer to an oxidizing gas; exposing the metal layer to a plasma-excited H2 gas; and exposing the metal layer to a plasma-excited Ar gas.

9. The method for smoothing a surface as claimed in claim 6, wherein the metal layer is selectively etched relative to the dielectric layer.

10. The method for smoothing a surface as claimed in claim 9, wherein the dielectric layer comprises SiO2 or a high-k material.

11. The method for smoothing a surface as claimed in claim 6, wherein the smoothing process uses an inductively coupled plasma (ICP) system.

12. The method for smoothing a surface as claimed in claim 6, wherein the kinetic energy of the plasma-excited Ar gas is controlled by applying an RF or pulsed DC bias voltage to the substrate.

13. The method for smoothing a surface as claimed in claim 6, wherein the metal layer system comprises copper.

14. The method for smoothing a surface as claimed in claim 6, wherein the smoothing process uses a rotating DC magnetic field system.

15. A plasma processing system comprising: a chamber including a substrate support configured to support a substrate, the substrate being planarized and including a metal layer and a dielectric layer on a horizontal plane; a gas injection system coupled to the chamber; a vacuum pump system coupled to the chamber; and a computer coupled to the gas injection system and the vacuum pump system, wherein the computer is configured to execute a program including instructions to perform the following: exposing the metal layer on the substrate to an oxidizing gas; exposing the metal layer to a plasma-excited H2 gas; and exposing the metal layer to a plasma-excited inert gas.

16. The plasma processing system of claim 15, further comprising a first RF generator coupled to the substrate support, wherein the substrate support serves as an electrode through which RF power is coupled to the plasma within the chamber.

17. The plasma processing system of claim 16, further comprising a rotating DC magnetic field system surrounding the chamber.

18. The plasma processing system of claim 16, further comprising: an upper plate electrode located on the substrate support; and a second RF generator coupled to the upper plate electrode.

19. The plasma processing system of claim 16, further comprising: an induction coil surrounding the chamber; and a second RF generator coupled to the induction coil.

20. The plasma processing system of claim 19, wherein RF power in a frequency range of 400 kHz to 100 MHz is inductively coupled from the induction coil to a plasma processing region within the cavity.

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