Patent Application
20130105331
The present invention concerns a photo/electrochemical inducing planarization and polishing technology based on confined chemical etching, especially the planarization and polishing methods and apparatus based on confined chemical etching on the surfaces of metals, semiconductors or insulates for the large-area and mass production.
Nowadays, in semiconductor industry, the feature linewidth of ultra-large scale integrate circuit (ULSI) has been down to 120 nm and the diameter of wafer has been up to 300 mm. Feature linewidth lower than 100 nm is coming into market. The number of transistors on a single chip has broken through 108. According to the development blueprint of the Semiconductor Industry Association of USA, the feature linewidth of microelectronics will be down to 50 nm, the wafer diameter will be up to 450 mm by 2011, and the metal layers of ULSI will be developed from 5-6 toward larger than 5-6. At present, the International Semiconductor Industry Association considers that the globe planarization of the wafer has to be performed when the feature size is down to 350 nm or less in order to ensure the precision and resolution of lithography. Planarization and polishing will become a key technique and play a crucial role in the development of the ULSI manufacture.
The precondition of USLI manufacture is that a substrate such as silicon wafer has to subject to the planarization and polishing of the surface and then is used. With the feature size of a device decreasing down to nanometer size, the requirements of surface quality, especially the surface planeness and mechanical damage of the silicon wafer become higher and higher while the resolution and the focusing depth of lithography have more and more limitation. Thus, the wafer with large size surface should have a surface with a profile precision of nanometer and a surface roughness of sub-nanometer. Meantime, the wafer should have a surface and subsurface without stresses and damages. Furthermore, each layer has to be planarized globally during the construction of the multi-layer wiring of ULSI. With the development of ULSI in the miniaturization of feature linewidth, three-dimensional constructions and multi-layer wiring, RC delay affects the performance of devices. In order to eliminate the RC delay, multi-layer high frequency metal interconnection structures have to be constructed. During the construction, if any layer constructing the device is too high in surface roughness, the device will have an increasing noise and poor uniformity of the electrical property, which will reduce frequency characteristic of the device and thereby reduce the integrated level, reliability and high quality product of ULSI. Thus, each layer for the device must be globally planarized, i.e., it is required to provide the removement and planarization of the excess deposition of copper (Cu) interconnectors of multiple wirings and of the dielectric layer such as SiO2, SiOF and other layer with superlow k having concavo and convex, which is a key step to achieve three dimensional structure of the ULSI. Moreover, as the feature size is further down, the adverse effect caused by diffusion of metal Cu into the SiO2 insulating layer will tend to be severe. It is an efficient way to provide a diffusion barrier layer or a porous delectric material with superlow k (insulator) in order to solve above technical problem. Moreover, Ta or TaN are considered a desirable metal material for the barrier layer due to their excellent adhesiveness to Cu and thermostability. However, the deposition of Ta or TaN onto the microcircuit board is non-selective. That means the insulator SiO2 layer will be covered totally. Thus, the unwanted Ta or TaN depositions have to be removed out of the Cu wire casing. Since Ta is a hard metal and is harder when oxidized, how to remove and planarize the unwanted Ta or TaN depositions will be another key technique for the globe planarization for the future ULSI manufacture. Moreover, how to planarize the fragile or flexible porous low-k dielectric material still remains great challenge. In summary, developing a multifunctional planarization and polishing method in nano-precision applied to metal, semiconductor and insulator at the same time is a common and key technique for the industrial manufacture of ULSI, which is recognized by all the countries all over the world.
Currently, chemical mechanical polishing (CMP) is the most effective technology and the only way to achieve the submicron size global planarization. CMP is a process of smoothing surfaces with the combination of chemical and mechanical forces, i.e., the oxidizers, catalysts and the like existed in the polishing slurry react with the surface atoms of the workpiece to form an oxide film on the workpiece surface, which is removed through mechanical friction by the abrasive grains suspended in the polishing slurry; the resulted fresh surface of the workpiece continues to be oxidized; thus, with the alternative oxidization and polishing, the workpiece surface is polished and planarized. However, due to the nonuniformity of mechanical friction and external force, the roughness after polishing still remains high, which will affect directly on the subsequent processes and also the finished product ratio. The most frequently used abrasive grains, e.g., Al2O3 particles, are hard and dispersed in the viscous slurry, which is easy to cause surface damages, even deep surface damages. Furthermore, there exist many other problems such as metal ion contaminations to workpiece surface, poor dispersion of abrasive grains in the slurry, and low removal rate in alkaline environment. During application of the CPM technology, another important issue is to determine and control the end points for the planishing-polishing method, i.e. it is difficult to determine when the desired amount of material has been removed or the desired degree of planarization has been obtained. If the oxide layer has been overly thinned and/or the desired degree of planarity has not been obtained during this process, the sequence process would be further done. More importantly, the future ULSI manufacture requires the wafer of having a profile precision of nanometer and a surface roughness of sub-nanometer, which is beyond the limit of CMP (in a 20 mm×20 mm area Ra<20 nm). Although, in recent years, assistant means of electrochemistry has be applied to CMP, where anodic dissolution is employed so as to improve the effects on polishing and planarization of copper or other semiconductor material. Unfortunately, the above mentioned disadvantages for CMP are not completely eliminated.
On the other hand, there are two development tendencies for the manufacture of modern precision optical devices: the micro-optical elements (MOC) and large dimensional ultra-precise optical devices. MOC is the optical element constructed by freeform optical surface with microstructure which has a profile precision of micrometer and a surface roughness of sub-nanometer, typically it includes the holographic lens, diffractive optical elements, GRIN-rod lens and so forth. With the tendency of miniaturization, MOC has prospective application into the science and technology of both national defense and civil industrial domains. Large dimensional ultra-precise optical devices are also important. For example, it may use as the huge lens and reflecting mirrors of space telescopes and the large dimensional high precise optical glass elements in laser fusion devices, which should have a surface roughness Ra of less than 1 nm in the high-frequency section. The polishing method for the ultra-precision optical devices is also dependent on CMP. Thus, it is necessary to develop a new planarization and polishing method which may avoid the disadvantages of CMP and, at meantime time, may achieve a profile precision of nanometer and a surface roughness of sub-nanometer.
The applicants, in their Chinese patent ZL03101271.X, disclose a fabrication method and its apparatus for 3D complex microstructures on metal surface. The method comprises: fixing a molded tool electrode with micro structure on a fixing mount; pouring the working electrolyte solution into an electrochemical cell; immersing the molded tool electrode into the working electrolyte solution; starting an electrochemical workstation to produce etchant on the surface of the molded tool electrode; confining a layer of the etchant into a micro/nano-meter thickness through a scavenger existed in the electrolyte solution; starting a manipulator to carry out a etching process for a workpiece by an etchant layer such that material of a surface of the workpiece is removed and separated from the etchant layer until the etching process is completed. The manufacturing apparatus includes a molded tool electrode, a fixed mount, a manipulator, an electrochemical workstation, and a computer control system. This method and its apparatus disclosed in the Chinese patent ZL03101271.X can perform the mass production of the duplication of various 3D complex microstructures. It is a one-step forming technique without coating, exposuring, developing and decoating. Consequently, the cost is lowered and the precision as well as the planarization are promoted dramatically. Since this technique is indeed distance sensitive, the removal amount for the workpiece can be well controlled by accurately controlling feeding distance of a template.
The first object of the present invention is to provide a photo/electrochemically-induced confined chemical etching method for the surface planarization and polishing in profile precision and surface roughness of nanometer scale. This method comprises the following steps: generating an etchant on the surface of a tool electrode having a nanometer-sized planeness through electrochemical, photochemical or photoelectrochemical means; the generated etchant reacting with a scavenger contained in a working electrolyte solution, or the etchant itself decaying such that an etchant liquid layer with a thickness of nanometer-scale is produced on the surface of the tool electrode; and the etchant in the etchant liquid layer reacting with a workpiece such that the surface of the workpiece is provided with nanometer-scale profile precision and roughness, so as to realize planishing and polishing in nano-precision for the workpiece.
The another object of the present invention is to provide an apparatus for the photo/electrochemically-induced confined chemical etching method for the surface planarization and polishing in profile precision and surface roughness of nanometer scale. The apparatus includes a tool electrode, a photo/electrochemical reaction control system, an optic control system, a working electrolyte solution recycling system, a working electrolyte solution temperature controlled system, and an automated computer-controlled system.
The tool electrode is a tool electrode having a planeness of nanometer-scale;
The photo/electrochemical reaction control system includes: a potentiostat, an optic control system, a photo/electrochemical working electrode, an auxiliary electrode, a reference electrode, a working electrolyte solution and a container. The tool electrode functions as the photo/electrochemical working electrode and connects with the potentiostat and/or the optic control system. The photo/electrochemical working electrode, the auxiliary electrode and reference electrode are immersed in the container which contains the working electrolyte solution and also contains the workpiece.
The working electrolyte solution recycling system is used to recycle the working electrolyte solution in the photo/electrochemical reaction control system;
The working electrolyte solution temperature controlled system is used to make the working electrolyte solution in the photo/electrochemical reaction control system at constant temperature;
The automated computer-controlled system includes: a fixed mount, a multi-dimension (>3D) micro-manipulator, a video monitor, a force sensor, a parallel laser ranging device, an electrical current feedback device and an information processing computer. The lower part of the fixed mount functions to fix the tool electrode, and the upper part of the fixed mount functions to connect with the Z-axial micro motor of the multi-dimension micro-manipulator in the automated computer-controlled system, wherein the Z-axial micro motor connects with the information processing computer. The container is provided on the X-Y-axial micro motors of the multi-dimension micro manipulator. The video monitor is used to monitor the approaching process of the tool electrode to the workpiece. The electrical current feedback device is used to monitor or measure the electrical current flowing through the surface of the tool electrode. The force sensor is used to detect whether the tool electrode touches the workpiece or not. The parallel laser ranging device is used to detect the distance between the tool electrode and the workpiece. According to the collected parameters such as the feedback current provided by the electrical current feedback device, the contact force provided by the force sensor, and the distance between the two surfaces provide by the parallel laser ranging device, the information processing computer send commands to the Z-axial micro motor and the X-Y-axial micro motors of the multi-dimension micro manipulator to adjust the distance and parallelity between the tool electrode surface and the workpiece surface.
The working electrolyte solution contains precursors of the etchant and/or leveling agent. The precursors of the etchant and/or leveling agent can generate the etchant and leveling agents on the surface of the tool electrode through photo/electrochemical reaction. If the generated etchant and leveling agents cannot decay spontaneously, the electrolyte solution further contains scavengers which can confine the etchant liquid layer on the tool electrode surface to a nanometer-scaled thickness.
The polishing and planishing method and apparatus disclosed by the present invention can realize a nanometer-scaled profile precision and surface roughness (i.e. nano-precision) on the surface of the workpiece. Comparing to CMP, the distinct advantages of the present invention lie in that: there is no surface and sub-surface mechanical damages for a workpiece; there is no residual stress; and the endpoint of polishing and planarization is easily controllable.
In the present invention, the term “photo/electrochemical” includes photochemical, electrochemical and/or photoelectrochemical means which are employed to produce etchant at a surface of the tool electrode.
In the present invention, the terms “etchant” or “etchant and/or leveling agent” refer to chemicals which can react with and etch the surface of the workpiece, or catalysts which can promote the etching and polishing rate of the etchant and/or leveling agent contained in the working electrolyte solution. The terms “precursors of the etchant” refer to materials which can produce an etchant through inducing of the photo/electrochemical reaction.
In the present invention, the term “confined etchant layer” refers to a liquid layer with a nanometer-sized thickness containing etchant and/or leveling agent which is generated through photo/electrochemical means and compacted by the scavengers in the working electrolyte solution. In general, the thickness of confined etchant layer is controlled no more than 100 nm. In the more accurate polishing and planarization process, it can be controlled less than 20 nm. The term “confined” means the thickness of the etchant liquid layer always keeps a thickness at nanometer scale during the processes of polishing and planarization of the workpiece.
In the present invention, the term “scavenger” is the chemical in the working electrolyte solution which can react with the etchant and/or leveling agent produced on the surface of the tool electrode and, consequently, compact the etchant liquid layer into a thickness of nanometer scale.
In the present invention, the terms “confined etching system” and “working electrolyte solution” refer to the solution system containing a precursors of etchant, a scavenger and optionally, a surfactant, a supporting electrolyte, a pH buffer, and the like according to special object to be etched, or, the solution system containing a precursors of etchant, and optionally, a surfactant, a supporting electrolyte, a pH a buffer, and the like according to special object to be etched. The term “mass balance of the confined etching system” means to keep a ratio of the components of the confined etching system in a reasonable range.
The present invention provides a photo/electrochemically-induced chemical etching method for surface planarization and polishing in a nanometer scale precision, characterized by use of the confined etchant layer technique (CELT). That is, through the photo/electrochemical means, a confined etchant and/or leveling agent layer having a thickness of nanometer scale is formed on the surface of the tool electrode with nano-planeness; the etchant reacts with and remove a workpiece material until the profile precision and surface roughness of the workpiece surface reach a precision of nanometer scale; thereby the workpiece achieves the desirable polishing and planishing treatment. The basic principle of CELT is described as followed:
(1) Through the electrochemical, photochemical or photoelectrochemical means, i.e. the suitable parameters such as potential or/and light wavelength and intensity is choosed based on chemical characteristics of the confined etching system, the precursors of etchant will be reacted to form etchant on the surface of tool electrode through the photo/electrochemical means. The etchant can be either a conventional chemicals or chemical radicals. The etchant generation reaction presented on the surface of the tool electrode can be expressed as:
R→O+ne or R+hν→O (+ne) (1)
where, R is the precursor of etchant and O is the generated etchant.
(2) The scavenger contained in the working electrolyte solution will react with the etchant in the vicinity of tool electrode surface to compact the etchant layer to a thickness of nanometer scale. If the generated etchant is a chemical radical, the thickness of the etchant layer can be compact through either the scavenger or spontaneous decay of the radical itself. The scavenging reaction can be expressed as:
O+S→R+Y or O→Y (2)
where, S is the scavenger and Y is product by a reaction between the etchant O and the scavenger or by the decay of the etchant O itself. The thickness of confined etchant layer depends on the reaction rate of reaction (1) and (2), and the concentration ratio of between the etchant precursor and the scavenger. If the concentration of the scavenger is pretty higher than that of the etchant precursor, a limited thickness of the confined etchant layer can be estimated by the following equation:
μ=(k2/DR)1/2 (3)
where, k2 is the quasi-first order reaction rate of scavenging reaction (2), DR is diffusion coefficient of etchant precursor in the working electrolyte solution. In the practical etching process, the thickness of confined etchant layer can be adjusted by changing the concentration ratio of between the etchant precursor and the scavenger, i.e., the precision of polishing and polarization can be tunable. Although the thickness of a confined etchant layer is at nanometer scale, the concentration distribution of etchant keeps constant inside the confined etchant layer.
(3) Through the automated computer-controlled system, let the confined etchant layer contacts with the workpiece and start the process of polishing and planarization for the workpiece. The etching reaction can be expressed as:
O+M→R+P (3)
where, M is the material of workpiece, and P is the product of a reaction between the workpiece and the etchant. The removal rate of the workpiece surface is determined by a rate of the etching reaction (3). Once the surface parts of workpiece contacting with the confined etchant layer are removed, the workpiece will separate from the confined etchant layer. Consequently, the polishing and planarization process stops. Then, the automated computer-controlled system will feed the tool electrode and continue the etching process until the workpiece is well polished and planarized.
Concretely, the photo/electrochemically-induced, chemical etching method for surface planarization and polishing in a nanometer scale precision described in the present invention includes the following steps:
(1) Fabricating a tool electrodes with nano-precision planeness surface as the working electrode for the photo/electrochemical reaction to produce an etchant through photo/electrochemical means;
(2) Fixing the tool electrode with nano-precision planeness surface on a fixed mount and a workpiece in the bottom of a container; or fixing the workpiece on a fixed mount (or the workbench), and fixing the tool electrode with nano-precision planeness surface in the bottom of a container;
(3) Pouring a working electrolyte solution into the container, which containing the etchant precursor, a scavenger, a solvent and at least one selected from the group consisting of a surfactant, a supporting electrolyte, a pH buffer and so on;
(4) Immersing the tool electrode (or the workpiece) into the working electrolyte solution as the working electrode for the photo/electrochemical reaction. In the container, the reference and auxiliary electrodes are also implanted. The photo/electrochemical reaction system starts and produces an etchant on the surface of tool electrode. The produced etchant reacts with the scavenger, or spontaneously decays if the etchant is free radical, and forms the confined etchant layer with a thickness of nanometer scale on the surface of tool electrode;
(5) Through the multi-dimension micro-manipulator driven by the automated computer-controlled system, feeding the tool electrode (or workpiece) and approaching it to the workpiece (the tool electrode); adjusting the distance and parallelity between the tool electrode surface and the workpiece with the help of in-situ observation from the video monitor and parallel laser ranging device. Furthermore, through controlling the distance between the tool electrode and the workpiece less than the thickness of the etchant layer or/and the relative motion (comprising translation movement and rotary movement) between the tool electrode and the workpiece to improve the mass balance of the working electrolyte solution. In this way, the uniformity of polishing and planarization is promoted, i.e., the profile precision of the working piece surface is improved and the surface roughness of the working piece surface is lowered;
(6) Feeding the tool electrode (or the workpiece) in vertical direction to the workpiece (or the tool electrode) such that the confined etchant layer on the tool electrolyte surface contacts with the workpiece surface, and thereby keeps the etching process advance. Also, the tool electrode or workpiece to be processed is fed in X-Y dimensions until the whole surface of workpiece to be processed is well polished and planarized.
In the step (4), the thickness of the confined etchant liquid layer can be tuned through controlling the potential of a tool electrode, a wavelength and intensity of a light-wave, and a formulation, temperature and recycle of the working electrolyte solution. The described confined etchant layer can be controlled at a thickness of 100 nm or less, preferably less than 20 nm. In practice, an amount of etchant is controlled by selection of the parameters such as the limiting diffusion potential of the generation reaction of the etchant as well as the wavelength and intensity of the excised light, and the selection of these parameters directly affect on the thickness of confined etchant layer, and the removal rate of the workpiece surface. The described “limiting diffusion potential” means under this potential the photo/electrochemical current keeps a constant maximum. For example, when etching Si under acidic condition, the “limiting diffusion potential” for oxidizing a bromide ion (Br−) to an etchant bromine (Br2) should be 0.9 V or more. The term “surface removal rate” refers the removal mass of workpiece surface material in a unit time by the chemical etching (g/min), or the removal thickness of the workpiece surface in a unit time (μm/min) For example, in the case of etching GaAs by Br2, the surface removal rate can be achieved 0.2 μm/min.
In the step (5), the distance and parallelity between the tool electrode and the workpiece can be controlled based on the feedback parameters provided by the current feedback device, the force sensor and the parallel laser ranging device.
The tool electrode (a working electrode in the photo/electrochemical system) used in the present invention has a nanometer-precise surface planeness, which can be planar and have a large size in X-Y dimensions, or be linear and have a large size in one dimension. The tool electrode may be fabricated by the method for preparing the tool electrode having a nanometer-precise surface planeness in the prior art, or may be fabricated by some new methods provided by the present inventor for preparing the tool electrode having a nanometer-precise surface planeness. These methods are listed as followed:
(1) Fabricating a tool electrode with nano-planeness of platinum, gold, iridium, tungsten or other metal though the ultra-precision machining (e.g., nano-cutting).
(2) Depositing a metal or semiconductor on a substrate with an atomic planeness through various processes for forming film (such as electron beam evaporation, magnetron sputtering, electroplating and chemical plating), or growing a metal or semiconductor on the substrate through crystal epitaxial growth technology to make the substrate conductive.
(3) Fabricating a single-crystal tool electrode having a surface of nano-planeness formed of platinum, gold, iridium or other metal through accurate cooling or drawing of the corresponding melt metal;
(4) Fabricating a electrode surface having a nanometer-planeness by polishing and planarizing polycrystal or single crystal metal electrodes through CMP technology together with an electrolytic process;
(5) Using a nanometer planeness surface formed by liquid metal or alloy spontaneously; and
(6) A process for photocatalytical or photoelectrocatalytical tool electrode, that is, the insulating quartz optical material or conductive ITO or FTO optical glass is used as a substrate, and is covered with a layer of TiO2, ZnO, WO3, Fe2O3, CdSe or the composite photocatalysts thereof through surface modification, electrochemical deposition in situ or chemical vapor deposition.
During the polishing and planarization process, the species produced in the vicinity of tool electrode surface through photo/electrochemical means may be either the etchant and leveling agent which can directly react with the material of workpiece surface, or be one of catalysts which can accelerate the chemical etching rate of the etchant and leveling agent contained in the electrolyte solution, or the both.
In the photo/electrochemically-induced confined chemical etching method for surface planarization and polishing in nanometer scale precision according to the present invention, the etchant and leveling agent can be widely chosen according to different workpieces. In general, etchants are determined by the material of workpiece, and suitable precursors of etchants and corresponding scavengers are determined by the etchants. Moreover, the selected precursors of the etchant and leveling agent and the selected scavengers are added to the working electrolyte solution, respectively.
The workpiece to be processed in the present invention can be a workpiese formed of metals (e.g., Cu in the USLI board), semiconductors (e.g., Si substrate), or insulators (e.g., SiO2 glass). Furthermore, the present invention can realize polishing and planarization in nano-precision of the surface of metals, semiconductors or insulators in mass production.
For example, when the workpieces to be processed are metals, the concentration of strong acids such as HNO3, H2SO4 and the like contained in the confined etchant layer is increased by a control of pH of the confined etchant layer, and thereby the metal is reacted with the acid to form soluble salts; or an etchant with strong oxidizability such as oxygen, fluorine, chlorine, bromine, ferrocene, iron cyanides, metal complex and the like is generated and can directly react with the objective metal to form soluble salt; or a strong oxidant such as hydrogen peroxide, oxygen, ozone, oxygen-containing radicals is generated and can react with the metal surface to form oxide, then, the formed oxide can be dissolved and removed by acid material contained in the electrolyte solution such as sulfuric acid, nitric acid, hydrochloric acid, oxalic acid and the like. The examples of the used scavengers are compounds containing disulfide bond such as cystine and the like, ferrocene and its derivatives, persulfates, nitrites, sulfites, thiosulfates, ascorbic acid, sorbitol, and mixtures thereof. In order to improve the solubility of metal ions in the working electrolyte solution, additives or surfactants may be added to the working electrolyte solution and complex with the metal cations. The examples of the additives and surfactants include halide ions, ammonia, cyanide ion, thiocyanide ion, crownether based supermolecular compounds, sodium alkylsulfonate, polyethers (preferably oxyethylene), nitrogen-containing azole based compound and so forth.
For example, when the objective workpieces are semiconductors such as Si, Ge, GaAs, or transition metals and their binary/ternary alloys thereof, or insulators such as quartz, glass, sapphire, MgO, TeCdHg, and KH2PO4, the strong oxidants such as oxygen, fluorine, chlorine, bromine, permanganate ion, bichromate ion, perchlorate ion, nitrate ion, or oxygen-containing free radicals are generated in the confined etchant layer, and meanwhile, complexing agents such as halide ions, ammonia, cyanide ion, thiocyanide ion, crownether based supermolecular compounds, sodium alkylsulfonate, polyethers (preferably polyoxyethylene), or nitrogen-containing azole based compound are added to the working electrolyte solution to improve the solubility of etching products and thereby achieve the object of polishing and planishing surface of the workpiece. The used scavengers are the reductive compounds such as cystine and the other compounds containing disulfide bond, ferrocene and its derivatives, persulfates, nitrites, sulfite, thiosulfates, ascorbic acid, sorbitol, or mixtures thereof. The used solvent may be water, acetonitrile, tetrahydrofuran, dimethylsulfoxide, N,N′-dimethylformamide, and ionic liquids including quarternary ammonium ion, quarternary phosphorus ion, imidazolium ion, or pyrrolium ion as a cation.
Moreover, as described above, the thickness of the confined etchant layer according to the present invention also depends on concentration ratio of etchant over scavenger. In general, the ratio is from 10:1 to 1:100, preferable 1:1 to 1:20. For example, when at least one selected from the group consisting of ferrocene and its derivatives, persulfate, dissolved oxygen, bromide, fluoride, and alkylamine is used as etchant precursor, the concentration of etchant precursor is usually controlled in a range from 0.001 to 1.0 mol/dm3. The etchant precursors generate etchants and/or leveling agents which can etch the surface of the workpiece through electrochemical reaction, and the generated etchants and/or leveling agents may rapidly react with at least scavenger selected from the group consisting of disulfide bond compounds, ferrocene and its derivatives, persulfates, nitrites, sulfites and thiosulfates. Consequently, the lifetime of the etchant and/or leveling agent will be shortened due to the fast scavenging reaction. Since their lifetime is very short, the etchant and/or leveling agents can't diffuse too far away from the tool electrode surface and would form a confined etchant liquid layer having a thickness of nanometer scale. That means the polishing and planarization can be processed with an extremely high resolution. Moreover, due to the existence of confined etchant liquid layer between the tool electrode and the workpiece, the tool electrode does not actually contact the surface of the workpiece and thereby keeps its surface retaining the nano-precise planeness.
When the etchant precursors used in the present invention are at least one selected from the group consisting of dissolved oxygen, ozone, peroxides, superoxides, oxynitrides, hypochlorites, nitrites, persulfates and alkyl peroxides, the concentrations of the etchant precursors are in range of from 0.001 to 1.0 mol/dm3. The free radical etchant is produced through photo/electrochemical reaction on the surface of tool electrode and etches the workpiece surface. Since the free radicals etchant in these cases are chemical radicals with very short lifetime, they only diffuse in the solution in very short diffusion distance from the surface of tool electrode. Thus, a confined etchant liquid layer having a thickness of nanometer scale may be formed on the surface of tool electrode. Thus, a chemical etching on the surface of the workpiece may be performed at extremely high resolution.
Moreover, due to the existence of confined etchant liquid layer between the tool electrode and the workpiece, the tool electrode does not actually contact the surface of the workpiece and thereby keeps its surface retaining the nano-precise planeness.
According, it can be concluded that the formulation of the working electrolyte solution used in the present invention meets the following requirements:
(1) The working electrolyte solution contains precursors of the etchants and/or leveling agents. The etchants and leveling agents can be produced through photo/electrochemical reaction and etch the workpiece surface. The reaction rate is sufficiently high, and the etching rate is not less than 10 nm/min.
(2) The chemical etching reaction between the etchant and the workpiece should be isotropic.
(3) There should be no indissolvable or poor dissolubility compounds formed during the process of polishing and planarization.
(4) The scavengers (confining agents) can react fast with the etchants and/or leveling agents and confine the etchant and/or leveling agent layer in a thickness of nanometer scale.
(5) The whole solution system should be stable.
Comparing to CMP, prominent advantages of the present invention are listed as followed:
(1) Since the tool electrode doesn't contact with the workpiece, there are no surface and subsurface mechanical damages and no residual stress on the workpiece surface.
(2) Since there are no other metal ions formed during the polishing and planarization process, which are easily precipitated, the metal ions contaminations can be avoided.
(3) Since the occurrence of a chemical etching and/or leveling reaction depends on whether the confined etchant liquid layer contacts with the workpiece surface or not, the polishing and planarization is typically a self-ended process. That means, with the chemical etching and/or leveling reaction going on, the surface material of workpiece is gradually dissolved and finally separates from the confined etchant layer. Then, the chemical etching reaction is stopped. Therefore, the end-point of polishing and planarization process is controllable.
The second object of the present invention is to provide an apparatus for the photo/electrochemically-induced confined chemical etching method which provides a workpiece with profile precision and surface roughness of nanometer scale. The apparatus includes a tool electrode, a photo/electrochemical reaction control system, a working electrolyte solution recycling system, a working electrolyte solution temperature control system, and an automated computer-controlled system.
The tool electrodes have a nano-precise planeness. The materials of the tool electrodes can be metals such as platinum, gold, iridium, tungsten, titanium and the like, and alloys thereof; or semiconductors such as TiO2, ZnO, WO3, Fe2O3, CdSe, and the like, and photocatalytic composites thereof; or non-metal materials such as glass carbon, high temperature pyrolysis graphite, boron doped diamond and so on.
There are no special limitation on the shape of tool electrode, which can be a milling cutter like shape, a linear scraper like shape, a planar disk like shape or a groove based on the principle of hydrodynamic design. However, reasonable designs of the shape of the tool electrode, relative motion between the tool electrode and the workpiece and feeding means of the tool electrode are selected such that the processed shape of the workpiece may be plane, cylinder, cone, sphere, and non-sphere. In this way, the profile precision of workpiece is improved, and the surface roughness of workpiece is lowered. For example, in the case of a disk tool electrode, the profile precision can be improved and the surface roughness can be lowered by means of keeping the tool electrode or workpiece rotating while lateral swing in a plane.
The photo/electrochemical reaction control system can precisely control the thickness of the etchant and/or leveling agent layer at nanometer scale. In general, the thickness of the confined etchant layer can be controlled no more than 100 nm. In the more precise process of polishing and planarization, the thickness of the confined etchant layer can be controlled no more than 20 nm. Herein, the photo/electrochemical reaction system includes: a potentiostat or/and an optic control system, a photo/electrochemical working electrode, an auxiliary electrode, a reference electrode, a working electrolyte solution and a container.
The working electrolyte solution recycling system includes a circulating pump, a flow controller and a fluid bath.
The working electrolyte solution temperature controlled system includes a thermoscope, a heater, a cooler and a fluid bath.
The automated computer-controlled system includes: a fixed mount, a multi-dimension (≧3D) micro manipulator, a video monitor, a force sensor, a parallel laser ranging device, an electric current feedback device and an information processing computer.
The tool electrode is fixed on the lower part of the fixed mount. The upper part of the fixed mount is connected with the Z-axial micro motor of the multi-dimension micro-manipulator in the automated computer-controlled system, wherein the Z-axial micro motor is controlled by the information processing computer. The tool electrode functions as the photo/electrochemical working electrode, which is connected with potentiostat and/or optical control system via the fixed mount. The auxiliary and reference electrodes are implanted into the working electrolyte solution in the container. The working electrolyte solution is heated to a constant temperature by the working electrolyte solution temperature controlled system, then pumped into the container, and circulated between the container and the fluid bath at constant temperature by the working electrolyte solution recycling system. The container is provided on the X-Y-axis micro motors of the multi-dimension micro-manipulator. The workpiece is placed in the container. The video monitor is used to control the procedure of the tool electrode approaching to the workpiece. The electrical current feedback device is used to detect the electrical current flowing through the surface of tool electrode. The force sensor is used to detect whether the tool electrode contacts with the workpiece or not. The parallel laser ranging device is used to detect the distance between the tool electrode surface and the workpiece surface. According to the collected parameters (such as a feedback electrical current provided by the electrical current feedback device, a contact force provided by the force sensor, and a distance between the two surfaces provided by the parallel laser ranging device), the information processing computer will send commands to the Z-axial micro motor and X,Y-axis micro motors of the multi-dimension micro-manipulator to adjust the distance and parallelity between the tool electrode surface and the workpiece surface. At the meantime, the multi-dimension micro-manipulator is also used to control the feeding means of tool electrode, and the relative motion means between the tool electrode and the workpiece.
The working electrolyte solution contains precursor of an etchant and leveling agent or/and a scavenger and a solvent, and optionally, a surfactant, a supporting electrolyte, a pH buffer and so on.
The solvent may be water, organic solvents, or ionic liquids. The organic solvent may be at least one selected form the group consisting of acetonitrile (CH3CN), dimethyl sulphoxide (DMSO), N,N′-dimethylformamide (DMF) and so on, and mixtures thereof. The ionic liquids include quarternary ammonium ion, quarternary phosphorus ion, imidazolium ion, pyrrolium ion or the like as a cation. The additives contained in the ionic liquids may be at least one selected from the group consisting of urea, acetamide, thiourea, trichloroacetic acid, phenylacetic acid, malonic acid, oxalic acid, p-toluene sulfonic acid, m-cresol, p-cresol, o-cresol, (-)fructose, and the like, and mixtures thereof. The additives have a concentration in a range of from 5% to 95% by mass based on the total mass of the working electrolyte solution.
The precursor of the etchant and/or leveling agent used in the present invention may be at least one selected from the group consisting of ferrocene and its derivates, persulphate, dissolved oxygen, bromide, fluid, alkyl amine compound and the like. In general, the precursor of the etchant and/or leveling agent has a concentration in a range of from 0.001 to 1.0 mol/dm3, preferably from 0.01 to 0.1 mol/dm3. The scavenger may be at least one selected from the group consisting of mercapto compound, ferrocene and its derivatives, persulfates, nitrites, sulfites, thiosulfates, and the like, and has a concentration in a range of from 0.05 to 10.0 mol/dm3, preferably 0.1˜1.0 mol/dm3. The surfactant may be at least one selected from the group consisting of alkylsulfonate, quarternary ammonium salts, polyethers, benzotriazole compound and the like. In general, the surfactant has a concentration in a range of from 0.001 to 1.0 mol/dm3, preferably 0.01˜0.1 mol/dm3.
The supporting electrolytes used in the working electrolyte solution function to ensure conductivity of the working electrolyte solution. The concentration of the supporting electrolyte is varied depending on the concrete material of the workpiece to be processed, and is usually 0.01 to 1 mol/L. A pH buffer added in the working electrolyte solution may be either acid or alkali.
The micro manipulator, the information processing computer, and the electrochemical reaction control system used in the apparatus for the photo/electrochemically-induced confined chemical etching method which provides a workpiece with profile precision and surface roughness of nanometer scale according to the present invention have been explained in the former Chinese patent ZL03101271.X of the present applicant, respectively. Based on the mentioned former patent, the micro manipulator extends from 3-dimensions to multi-dimensions (≧3D), which is suitable for the polishing and planarization of complex surface types. Meantime, an optical control system is added in the apparatus of the present invention so as to modulate the process of polishing and planarization in photo- and electrochemical means. Since a confined etchant layer technique is adopted and a thickness of the confined etchant layer formed by photo/electrochemical reaction on the surface of tool electrode is at nanometer scale, the parallelity between the tool electrode and the workpiece is very crucial, especially, in the process of polishing and planishing a workpiece with large area. In the former patent, a force sensor is adopted to adjust the distance between the tool electrode and the workpiece. In the present invention, a parallel laser ranging device is further incorporated into the apparatus of the present invention so as to adjust the distance and parallelity between the tool electrode and the workpiece together with the force sensor. In addition, the automated control system is incorporated into the apparatus of the present invention for fine tuning the multi-dimensional micro-manipulator, and, for feeding the tool electrode toward the workpiece until the confined etchant layer contacts the surface of workpiece to start the etching process through the feedback information from the current feedback device, the force sensor and the parallel laser ranging device. With the etching and leveling process going on, the surface material of workpiece is gradually dissolved and removed, and the automated control system keeps feeding the tool electrode through the multi-dimensional micro manipulator until the polishing and planarization well-done. The polishing and planarization process may be monitored through the video monitor. In order to ensure complement of scavenger and the mass balance during the polishing and planarization process, and in order to adjust etching reaction rate, the working electrolyte solution recycling system and the temperature control system are added in the present invention.
There is no mechanical stress, surface and subsurface damage, and metal ion contamination on the workpiece surface during the polishing and planarization process. The processing end-point of the polishing and planarization process is very easily controlled. Meanwhile, the method according to the present invention may apply to the mass production of large scale surfaces of various materials such as metals, semiconductors and insulators.
In the apparatus shown in
The photo/electrochemically-induced, confined chemical etching method for surface planarization and polishing in profile precision and surface roughness of nanometer scale as described in the present invention, can not only perform the operation of polishing and planarization on the large-scale area, but also work on the workpieces with 2-D planar, cylindrical or irregular surface.
As shown in
As shown in
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As shown in
Next, the present invention will be more fully understood by reference to the following Examples together with other Figures. However, the Examples are only illustrative, and should not be construed as limited the scope of the invention.
The surface of a Cu workpiece was planarized and polished using the apparatus shown in
The surface of a Si workpiece was planarized and polished using the apparatus shown in
The surface of a gallium arsenide (GaAs) workpiece was planarized and polished using the apparatus shown in
The SiO2 glass surface of a workpiece was planarized and polished using the apparatus shown in
Processing steps in examples 5 to 13 were similar to those in example 2, processing steps in examples 14 to 22 were similar to those in example 1, and processing steps in examples 23 to 31 were similar to those in example 4, except that the components of the workinging electrolyte solutions, of the scavengers, and contents thereof. The specific components and contents thereof may be seen in tables 1 to 3. Etching results can be characterized using high-powered optical microscope (Olympus BX-51, manufactured by Olympus Co.) and atomic microscope (Tapping Mode Nanoscope IIIa, manufactured by Digital Instrument Co). The surface roughness Ra was obtained by analysis the scanning results from atomic force microscope (Tapping Mode Nanoscope IIIa, manufactured by Digital Instrument Co).
As shown in Table 1, the nano-precision photo/electrochemical planarization and polishing methods according to the present invention can be applied in the polishing of Si, and result in remarkable effect. Moreover, surface roughness Ra of the resulted workpiece was 2.0 nm or less.
As seen in Table 2, the nano-precision photo/electrochemical planarization and polishing methods according to the present invention can be applied in the polishing of Cu workpiece, and result in remarkable effect. Moreover, surface roughness Ra of the resulted workpiece was 25 nm or less.
As seen in Table 3, the nano-precision photo/electrochemical planarization and polishing methods according to the present invention can be applied in the polishing of SiO2 workpiece, and result in more remarkable effect. Moreover, surface roughness Ra of the resulted workpiece was 1.5 nm or less.
The planarization and polishing method and an apparatus therefor the same according to the present invention may provide a surface of the workpiece with profile precision and surface roughness of nanometer scale (i.e nano-precision). In addition, the present invention has more advantages over CMP technology due to no surface and subsurface mechanical damage, no residual stress, no metal ion contaminations, and having controllable end-point of the polishing and planarization process. Therefore, the present invention is especially suitable to be applied in the field of the industrial semiconductor materials, ultra large scale integrated circuit, Micro Electromechanical System (MEMS), Micro Opto-Electro-Mechanical Systems, modern precision optical device, aerospace engine blades, and etc..
| Number | Date | Country | Kind |
|---|---|---|---|
| 201010219037.5 | Jul 2010 | CN | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/CN11/76700 | 6/30/2011 | WO | 00 | 1/4/2013 |
