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Process for low temperature, dry etching, and dry planarization of copperRelated Patent Categories: Semiconductor Device Manufacturing: Process, Chemical Etching, Vapor Phase Etching (i.e., Dry Etching), Utilizing Electromagnetic Or Wave Energy, By Creating Electric Field (e.g., Plasma, Glow Discharge, Etc.)Process for low temperature, dry etching, and dry planarization of copper description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060014394, Process for low temperature, dry etching, and dry planarization of copper. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS-REFERENCE TO RELATED APPLICATION [0001] This application is a continuation of U.S. patent application Ser. No. 10/285,179, filed Oct. 31, 2002, which claims the benefit of U.S. provisional patent application Ser. No. 60/335,641, filed Oct. 31, 2001, which are hereby incorporated by reference in their entirety. BACKGROUND [0002] Background for the subject technology can be found in the pioneering work by the late Carl Wagner [32] on the active-passive oxidation of Si and the work by Lou, Mitchell and Heuer [33] on treatment of volatility diagrams. [0003] The replacement of aluminum with copper as an interconnect material in ULSI structures is being actively considered in the semiconductor industry [1-7]. Compared to aluminum, copper has a lower electrical resistivity and significantly larger electromigration resistance. The utilization of copper interconnects is expected to improve chip performance due to lower RC time delays and power dissipation. Two basic processes for copper-based interconnect structures have been proposed [2-6]. The first is the dual damascene method, which has been recently introduced in IC manufacturing by IBM and others [2]. In this method for producing multilevel structures, the dielectric layer is first deposited on the silicon substrate. The via pattern is then formed by dry etching of the dielectric. A thin diffusion barrier (e.g., TaN) is subsequently deposited on the patterned dielectric followed by deposition of copper using blanket CVD, PVD or electroplating techniques. The excess copper on top of the features is finally removed by a process of chemical mechanical polishing (CMP). [0004] The second process for the patterning of copper is the dry etch method [3-5], which is based on the process used for the patterning of aluminum [8-10]. In this process, copper is first deposited on the dielectric having a barrier layer using, for example, a sputtering process. A hard mask or photoresist is applied over the copper and the pattern is etched using a dry etch process such as reactive ion etching(RIE). The structure is then filled with dielectric and planarized using CMP. [0005] Unfortunately, unlike aluminum, the anisotropic dry etching process for copper using halides is difficult at low temperatures due to the lower volatility of copper based compounds [5, 11-14]. At wafer temperatures of 200.degree. C. or higher, reactive ion etching of copper in the presence of chlorine can be achieved, but the anisotropy is lost due to problems associated with the use of organic mask layers. Inorganic mask layers have been utilized, however they appear to provide less etch anisotropy due to the lack of protective sidewall films. Higher temperatures have a negative effect on the electrical conductivity of copper due to the increased solubility of chlorine in copper and can also cause significant problems due to dopant diffusion and stress on equipment and components during manufacturing. Redeposition of the copper chlorides from the warm exhaust gas stream on cooler equipment components is a serious issue associated with high temperature etching. The continued corrosion of the patterned copper due to the precipitation of copper chloride compounds (15) on cooling to room temperature is yet another problem. Thus lower etching temperatures are desirable. [0006] The continued corrosion of the patterned copper during the etch process itself has been reported to be an important problem. One would expect that this problem can be easily resolved by ensuring that the nonvolatile copper chlorides are removed on completion of the etch to prevent further corrosion. However this problem needs to be considered carefully. The existing dry etch processes for Cu are usually carried out at higher temperatures (typically >200.degree. C.). Hence the solubility of chlorine in copper is much higher or alternatively the diffusion depth of chlorine in copper is significant compared to lower temperatures. On cooling down to room temperature, the solubility of chlorine in copper is reduced and as a result the excess chlorine may precipitate in the form of copper chlorides causing further corrosion. To prevent this from occurring, the residual chlorine in the etched copper has to be removed by exposing to a reducing atmosphere such as hydrogen or by further etching of the copper containing the dissolved chlorine without the use of chlorine, in effect using natural volatilization (a process which is extremely slow). Such a process besides causing problems with dimensional control would require a significant amount of time, since it is known that the diffusivity of chlorine in copper is very low (perhaps more than two orders of magnitude lower than that of copper at room temperature [15]). A dry etch process for copper at lower temperatures besides being beneficial in relation to the thermal cost, has the advantage that the diffusion depth of chlorine in copper is reduced, and hence corrosion problems due to the reasons stated earlier are expected to be less severe. Hence, the development of a cost-effective, low-temperature, dry etch process for copper is highly desirable. This appears to be elusive at the present time. [0007] An important advantage of the damascene process is that the copper does not have to be patterned directly and so the problems associated with the dry etching of copper are avoided. The number of processing steps, as compared to the dry etching process, is also reduced [2]. However, there are some challenging issues associated with the Cu deposition for very small feature sizes (less than 0.25 .mu.m) and high aspect ratios [16,17] as well as difficulties associated with the copper CMP process [16,18-19] and the safe disposal of the byproducts of the electroplating process [20,21]. The practical problems and costs associated with the transition to the new technology based on the copper damascene process are still being evaluated with copper CMP regarded as the major bottleneck [16,19,22]. Furthermore, the introduction of porous, low dielectric constant materials is expected to pose significant challenges to the copper CMP process in the near future [18]. [0008] In comparison, if a cost-effective, low temperature dry etching process for copper can be developed, many of the problems associated with copper damascene can be avoided. Besides, the transition may be relatively easier and cheaper since essentially the same equipment and processes used for the patterning of aluminum can be utilized. An obvious benefit of the dry process, as compared to the damascene process, is that there is considerable experience in handling of the disposed gases using safe and environment-friendly procedures. It has been recently reported [12] that dry etch patterning can be readily combined with air gaps, which is an important advantage over the damascene process, since air gaps reduce capacitance and leakage current between lines. An added benefit of this innovation is that no barrier films on the sidewalls of the copper lines are needed. [0009] In spite of the difficulties with copper reactive ion etching, several approaches have been proposed. Most of these utilize temperatures over 150.degree. C. to improve the volatility of the copper chloride gas species. Winters [23] suggested a two-step process, that first required the formation of CuCl at room temperature followed by heating between 150-200.degree. C. to desorb Cu.sub.3Cl.sub.3(g), which he identified as the primary desorbing species. Arita et al. [5] and Igarashi et al. [24] used a gas mixture of SiCl.sub.4, Cl.sub.2, N.sub.2, and NH.sub.3 for dry etching and used a resist mask by hard-baking above 250.degree. C. Since copper was oxidized during the ashing process for resist removal, they suggested a CVD SiO.sub.2 film or a plasma CVD nitride mask which could remain in place after the RIE. A thin SiON film that formed on the sidewall acted as a suitable barrier during the etching process. Miyazaki et al. [14] were able to obtain an anisotropic etch profile using chlorine as the only reactant. The operating temperature (230-270.degree. C.) and the partial pressure of chlorine (0.3-1.3 Pa) had to carefully controlled in their process. Markert et al. [12] used an ICP system containing a chlorine-based gas mixture (10-20 mtorr) along with Ar and N.sub.2 at a wafer temperature of 250.degree. C. CH.sub.4 was used to protect the sidewall during the etch. Ye et al. [25] utilized a mixture of HCl or HBr along with hydrogen as the reactant gases at temperatures greater than 150.degree. C. Jain et al. [26] used a two-step process for the isotropic etching of copper, first by oxidizing copper using hydrogen peroxide followed by its removal using hexafluoroacetylacetone (hfacH). A continuous process using the same reactants at 150.degree. C. was also proposed by them. Lee et al. [27] performed a RIE etch using a CCl.sub.4/N.sub.2 electron cyclotron plasma (ECR) at temperatures above 210.degree. C. [0010] Low temperature methods for the etching of copper usually require some form of radiation such as ultraviolet [28-29], infrared [30] or laser [31], to enhance the volatilization of the etch products. It is reported that these do not have good etch uniformity for large-area substrates and the processes are not easy to control and maintain [32]. Kuo and Lee [32, 33], and Allen and Grant [34] suggested a process in which CuCi.sub.x was intentionally formed by exposing to a chlorine plasma and then chemically etched using HCl or other solutions [34]. Temperatures ranging from 25-250.degree. C. were utilized by Kuo and Lee with the higher temperatures being more effective. Surface roughness along the sidewalks appears to be an important issue in their process. [0011] One of the problems in understanding the etching process in the Cu--Cl system is that it is not clear which Cu gaseous species is responsible for the primary etching mechanism. It has been reported that the trimer Cu.sub.3Cl.sub.3 is the dominant gas species in this system [5,23], however the precise conditions (i.e., chlorine partial pressure and temperature) for which this species dominates appears to have not been analyzed systematically. The difficulty arises because there are a large number of gaseous (Cu(g), CuCl(g), CuCl.sub.2(g), Cu.sub.3Cl.sub.3(g)), etc.) and condensed (solid) phase species (Cu(c)*, CuCl(c), CuCl.sub.2(c)) in this system. Experiments involving mass spectrometric measurements of the partial pressures of gases along with other techniques to determine the condensed phase species are certainly helpful, however these can be quite tedious. Theoretical approaches based on the thermodynamics of this system may be preferable provided the thermodynamic data is available. Fortunately, for the Cu--Cl system, the thermodynamic data has been assessed and is available through a number of sources, for e.g., JANAF tables [35] or databases accompanying thermodynamic software such as HSC Chemistry [36] and FACT [37]. In spite of this, it is still not a trivial task sorting through the list of reactions and determining the dominant condensed and vapor phase species at various temperatures and chlorine partial pressures. A graphical representation is probably the best approach. Conventional phase diagrams, Richardson-Ellingham diagrams and Pourbaix diagrams [38] are a few examples of such graphical representations that are useful in various areas of research. In the present situation, an appropriate graphical method for representation of solid-gas reactions, so as to understand etching mechanisms, is a "volatility diagram." Volatility diagrams are typically used in the high-temperature industry to examine volatility behavior of materials such as refractories and ceramics when exposed to high temperatures and reactive environments [39-44]. The earliest works in this field appear to be those of Wagner (39) who utilized such a diagram for analyzing the active-passive oxidation of silicon. Gulbransen and Jansson (40-42) used these diagrams (also known as thermochemical diagrams) for the analysis of volatility behavior of refractory metals (40), ceramics (41) and liquid metals (42). Typically, in a volatility diagram, for example the Si--O system, the partial pressures of the important volatile species that contain the solid element (e.g., Si(g), SiO(g)) are plotted as a function of the partial pressure of oxygen at various temperatures. A comprehensive review of the construction and application of volatility diagrams for ceramic materials is given in the paper by Lou, Mitchell and Heuer (43, 44). The present work extends the use of volatility diagrams for understanding dry etching mechanisms in the Cu--Cl system. *c--crystal, l--liquid, g--gas SUMMARY OF THE INVENTION [0012] The subject invention pertains to a method and apparatus for etching copper (Cu). The subject invention can involve passing a halide gas over an area of Cu such that CuX, or CuX and CuX.sub.2, are formed, where X is the halide. Examples of halides which can be utilized with the subject matter include, but are not necessarily limited to, Cl, Br, F, and I. Once the CuX, or CuX and CuX.sub.2, are formed the subject invention can then involve passing a reducing gas over the area of Cu for a sufficient time to etch away at least a portion of the CuX or CuX.sub.2, respectively. With respect to a specific embodiment in which CuX and CuX.sub.2, are produced when the halide gas is passed over the area of Cu, the reducing gas can be passed until essentially all of the CuX.sub.2 is etched and at least a portion of the CuX is etched. Examples of reducing gases which can be utilized with the subject invention include, but are not necessarily limited to, hydrogen gas and hydrogen gas plasma. The subject invention can accomplish the etching of Cu by passing the reducing gas over the Cu so as to be on a CuX.sub.2--Cu.sub.3X.sub.3(g) metastable line when etching CuX.sub.2 and to be on a CuX--CuY(g) metastable line, where Y is the reducing gas element, when etching CuX. FIGS. 5, 6, and 8, show such metastable lines for Cu, with X being Cl, from temperatures ranging from about 50.degree. C. to 200.degree. C. These can be extrapolated to other temperatures, for other halides, and/or other reducing gases. The subject invention can be used to, for example, etch partial into a layer of Cu, through a layer of Cu, or to smooth a Cu surface. [0013] The volatility diagram for Cu--Cl was constructed at temperatures between 50 and 200.degree. C. based on the procedure developed by Lou, Mitchell and Heuer [43,44]. Examination of the volatility diagram revealed that Cu.sub.3Cl.sub.3(g) had the highest vapor pressure. However the equilibrium vapor pressure of Cu.sub.3Cl.sub.3(g) is not sufficient for the purpose of dry etching of copper at temperatures below 200.degree. C. At temperatures greater than 200.degree. C. and with chlorine pressures lower than that given by the isomolar point (FIG. 3), it is possible to etch copper without the formation of CuCl(c), due to the active oxidation of copper involving the Cu(c)-Cu.sub.3Cl.sub.3(g) vaporization reaction. The subject invention relates to the etching of copper at low temperatures (25.degree. C.) by using the metastable CuCl.sub.2(c)-Cu.sub.3Cl.sub.3(g) volatilization reaction in the presence of hydrogen (FIGS. 5, 6). In a specific embodiment, the subject invention relates to a multi-step etch process (FIG. 10), which first includes the rapid and preferential formation of CuCl.sub.2(c) followed by its removal using the above reaction. The successful implementation of this low-temperature, dry etch process can provide a rapid, cost-effective and environment-friendly alternative to the Cu damascene process. A further application of this multi-step process for the reactive ion planarization (RIP) of copper may offer an attractive alternative to the chemical mechanical planarization (CMP) step in the damascene process. DESCRIPTION OF THE DRAWINGS [0014] FIG. 1 is a volatility diagram for the Cu--Cl system at 50.degree. C. where the gas phase species are considered individually: (a) Cu(g), (b) CuCl(g), (c) CuCl.sub.2(g) and Cu.sub.3Cl.sub.3(g). Partial pressures for all figures are given in atm (1 atm=1.013.times.10.sup.5 Pa=760 torr). The reaction numbers for the vapor pressure lines are also indicated (see Table 2). [0015] FIG. 2 is a complete volatility diagram for the Cu--Cl systems at 50.degree. C. Pressures are given in atm. The maximum equilibrium vapor pressure line segments are solid, the stable portions of the equilibrium vapor pressure lines that are less than the maximum are dashed, and the metastable extensions are dotted. The two vertical lines separate the Cu(c), CuCl(c) and the CuCl.sub.2(c) condensed phase regions. The dominant gas species are: Region I--Cu(g), Region II--CuCl(g), Region III--Cu.sub.3Cl.sub.3(g), and Region IV--CuCl.sub.2(g). The vapor pressure of Cu.sub.3Cl.sub.3(g) over CuCl(c) (approx. 10.sup.-18 atm) is the maximum attainable in this system (DH), however since it is significantly less than 10.sup.-8 atm, dry etching of copper under equilibrium conditions is not possible at this temperature. [0016] FIG. 3 is a master volatility diagram for the Cu--Cl system from 50 to 200.degree. C. The vapor pressures of all the gas species indicated in the figure (in atm), increase with temperature and at temperatures greater than 200.degree. C., the vapor pressure of Cu.sub.3Cl.sub.3(g) is sufficiently high (10.sup.-8 atm) to be effective for dry etching of copper. The isomolar line for the reaction involving the active oxidation of Cu, i.e., 3Cu(c)+ 3/2Cl.sub.2(g)=Cu.sub.3Cl.sub.3(g), is given by the mass balance relation P.sub.Cu.sub.3.sub.Cl.sub.3.sub.(g)=2/3P.sub.Cl.sub- .2.sub.(g), and is indicated in the figure. At the isomolar point, the vapor pressure of Cu.sub.3Cl.sub.3(g) as given by the mass balance relation, is equal to the equilibrium vapor pressure for the reaction 3CuCl(c)=Cu.sub.3Cl.sub.3(g), a the specific temperature. Note that the conversion of copper to copper chlorides at lower temperatures (50.degree. C.) occurs at lower partial pressures of chlorine as compared to higher temperatures (200.degree. C.). [0017] FIG. 4 is an isomolar line obtained by connecting the isomolar points (filled circle) for the reaction 3CuCl.sub.2(c)=Cu.sub.3Cl.sub.3(g- )+ 3/2Cl.sub.2(g), on the volatility diagram. At the isomolar point, both the mass balance criterion (P.sub.Cu.sub.3.sub.Cl.sub.3.sub.(g)=2/3P.sub.- Cl.sub.2.sub.(g)) and the termodynamic equilibrium governed by the above reaction, are satisfied at the given temperature. The isomolar line defines the maximum partial pressure of Cu.sub.3Cl.sub.3(g) in a non-reactive environment (i.e., with Cl.sub.2(g) as the only external gas). Note that the isomolar point in FIG. 3 is relevant for the active oxidation of Cu and falls on CuCl(c)/Cu.sub.3Cl.sub.3(g) vapor pressure line. The mass balance criterion and hence the isomolar line is however the same in both cases. [0018] FIG. 5 is a volatility diagram for the Cu--Cl system showing the isobaric lines for various partial pressures of hydrogen (in atm) for the reaction 3CuCl.sub.2(c)+ 3/2H.sub.2(g)=Cu.sub.3Cl.sub.3(g)+3HCl(g). The isobaric lines are obtained by joining the isobaric points (symbols), which indicate the maximum vapor pressure of Cu.sub.3Cl.sub.3(g) over CuCl.sub.2(c), allowed for a given hydrogen partial pressure at a specific temperature. With reference to the above reaction, the isobaric point is obtained with the assumption P.sub.HCl=3P.sub.Cu.sub.3.sub.Cl.su- b.3. It is clear that very high partial pressures of Cu.sub.3Cl.sub.3(g) are achievable at low temperatures in a hydrogen-based reducing environment, hence higher etch rates for copper can be obtained. [0019] FIG. 6 is a volatility diagram for the Cu--Cl system showing the isobaric lines for various partial pressures of atomic hydrogen (in atm) for the reaction 3Cu.sub.3Cl.sub.2(c)+3H(g)=Cu.sub.3Cl.sub.3(g)+3HCl(g). The isobaric lines are obtained in a manner similar to FIG. 5 using the same mass balance criterion P.sub.HCl=3P.sub.Cu.sub.3.sub.Cl.sub.3. The isobaric lines have a negative slope, which is in contrast to the isobaric lines in FIG. 5. As a result, the Cu.sub.3Cl.sub.3(g) pressure for a given pressure of atomic hydrogen, H(g), decreases as the temperature is increased. Extremely high Cu.sub.3Cl.sub.3(g) pressures over CuCl.sub.2(c) are obtained in this atomic hydrogen environment at low temperatures, as a result the etch rates for CuCl.sub.2(c) and hence copper are expected to be high. [0020] FIG. 7 shows isobaric lines for the Cu(g)-CuCl(c) metastable equilibrium line using (a) H.sub.2(g) and (b) H(g). The vapor pressures are not sufficient enough (less than 10.sup.-8 atm) to cause noticeable etching of CuCl(c) at low temperatures. Continue reading about Process for low temperature, dry etching, and dry planarization of copper... Full patent description for Process for low temperature, dry etching, and dry planarization of copper Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Process for low temperature, dry etching, and dry planarization of copper patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. 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