Abstract:
A process for, and apparatus for, Chemically-Mechanically Polishing (CMP) a semiconductor wafer with a slurry including ElectroRheological (ER) and/or MagnetoRheological (MR) fluids. The combination of the materials and an electric field provides inherent tuning of polishing rates, locally and globally, and improves flatness and uniformity, as well as minimizing recession and erosion.

Description:
FIELD OF THE INVENTION 
     This invention relates to an integrated circuit manufacturing process and an apparatus therefor and in particular to a method and apparatus for polishing of metal layers during fabrication of integrated circuits. 
     BACKGROUND OF THE INVENTION 
     A. Polishing of metal layers in IC processing 
     Chemical Mechanical Polishing (CMP) includes both chemical reaction and mechanical abrasion. It involves the use of a polishing slurry which contains chemically reactive components which form soft compounds with the surface region of the material being polished. The slurry also contains abrasive particles which mechanically preferentially remove the softer reacted surface region when the wafer is moved across the polishing pad having slurry thereon. 
     As integrated circuits become smaller, higher density, and faster, the process technology for manufacture of these circuits has undergone major transitions. With device critical dimensions decreasing below 0.25 microns, a major factor in limiting circuit response times is the metal interconnections connecting devices and circuit elements. Hence, decreasing the resistance of the metal lines, and reducing the capacitance between metal lines, are of great importance in IC process development. Two areas of development have been: 1) Cu metallization used in place of Al or Al alloy metallization due to the lower resistivity and higher electromigration resistance of Cu, and 2) use of low dielectric constant (low K) dielectrics as insulation between metal layers to reduce the capacitive coupling. 
     Use of Cu in IC metallization presents challenges, including the difficulties in reactive ion etching (RIE) of Cu. An alternative strategy to eliminate the requirement of RIE for Cu metallization is to use Damascene structures. This technique involves depositing an insulating layer, patterning and etching trenches in the insulation where the metal lines are to be placed, depositing Cu over the whole surface and into the trenches, and finally polishing off the surface Cu using Chemical Mechanical Polishing (CMP), thereby leaving only the embedded Cu lines in the trench regions. Related methods such as dual Damascene can integrate via and interconnect formation. 
     Current CMP technology requires refinement of polishing methods in order to avoid problems such as recession and erosion, as illustrated in FIGS. 1 a  and  1   b . Current CMP technology also exhibits other types of Within-Wafer Non-Uniformity (WIWNU) of polish rate which result in uneven material removal across the wafer. 
     B. Field responsive materials 
     Field responsive materials exhibit a rapid, reversible, and tunable transition from a liquid-like, free-flowing state to a solid-like state, upon the application of an external field. These materials demonstrate dramatic changes in their rheological behavior (viscosity) in response to an externally applied electric or magnetic field, and are known as electrorheological (ER) fluids or magnetorheological (MR) fluids respectively. ER fluids may include linear dielectric particles (such as: silica, alumina, titania, barium titanate, semiconductors, weakly conducting polymers, zeolites, polyhydric alcohol, sodium carboxymethyl Sephadex, alginic acid, carboxymethyl Sephadex, lithium hydrazinium sulfate, or combinations thereof) colloidally dispersed in nonconducting continuous phase liquids (such as: silicone oils, mineral oils, paraffin oils, hydraulic oils, transformer oils, perfluorinated polyethers, or combinations thereof). Alternatively, homogeneous liquid-crystalline polymer-based materials are known, as reported by Inoue et al in the MRS Bulletin, August 1998. MR fluids comprise ferromagnetic or ferrimagnetic, magnetically nonlinear particles (such as: iron, iron alloys, iron oxide, iron nitride, iron carbide, carbonyl iron, low carbon steel, silicon steel, chromium dioxide, fumed or pyrogenic silica, silica gel, titanium dioxide, magnetite, nickel, cobalt, manganese, zinc, ceramic ferrites, or combinations thereof) dispersed in an organic or aqueous continuous phase liquid (such as: H2O, silicone oils, kerosene, mineral oils, paraffin oils, hydraulic oils, transformer oils, halogenated aromatic liquids, halogenated paraffins, diesters, polyoxyalkylenes, fluorinated silicones, cornstarch, olefin oil, glycol, or combinations thereof). Both ER and MR fluids may additionally comprise surfactants (which act as wetting agents) and thixotropic additives (which make the particles hydrophilic). Details of continuous phase liquids, particulates, surfactants, and thixotropic additives used for MR and ER fluids are found in US Patents having the following serial numbers: U.S. Pat. No. 5645752, U.S. Pat. No. 5167850, U.S. Pat. No. 4992190, U.S. Pat. No. 4033892, USRE032573, U.S. Pat. No. 3917538, and U.S. Pat. No. 4772407. All of these aforementioned patents are hereby incorporated by reference. 
     A feature shared by the ER and MR fluids is that after an external field is applied, the material rapidly transforms from a fluid into a weak viscoelastic solid, generally through the formation of chains and columns of the field responsive particles. These field-induced chain-like structures possess a non-zero shear modulus and a shear stress. 
     ER and MR materials are of interest in, and have been investigated for, such applications as engine mounts, shock absorbers, clutches, seat dampers, variable-resistance exercise equipment, earthquake-resistant high-rise structures, positioning devices, and optical polishing of aspherical surfaces. The materials science of field-responsive fluids is described in the MRS Bulletin, August 1998. 
     Optical glass polishing using MR fluids has been described by Jacobs et al in U.S. Pat. No. 5,616,066, issued Apr. 1, 1997, and in U.S. Pat. No. 5,795,212, issued Aug. 18, 1998. According to these patents, portions of the glass workpiece are selectively polished by moving the workpiece through a work zone having a stiffened MR fluid therein and therefore a high pressure. The MR finishing machine comprises an electromagnet, a trough for MR fluid containment, and a work spindle to which the curved glass workpiece is mounted. 
     ER and MR materials have not, to the best knowledge of the inventor, been utilized or investigated in the prior art for the polishing of metal layers in semiconductor processing. It is believed that their utilization for this application will enable solutions to some of the aforementioned problems which exist in current Al and Cu CMP technology, including recession and erosion, and other types of WIWNU. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of this invention to provide a semiconductor manufacturing process for metal CMP and an apparatus therefor utilizing ER and/or MR materials. 
     It is a further object of this invention to provide a semiconductor manufacturing process for metal CMP and an apparatus therefor which is self-adjusting to provide maximum flatness across the wafer. 
     It is a further object of this invention to provide a semiconductor manufacturing process for metal CMP and an apparatus therefor which prevents recession and erosion of metal features. 
     These objects are met by using ER and/or MR materials as one component of CMP slurry, and by providing a method and apparatus for applying appropriate electric or magnetic fields across the ER and/or MR materials to cause their viscosity to self-adjust and alter the local polishing rates so as to yield a uniform flat surface across the wafer. 
    
    
     SUMMARY OF THE FIGURES 
     FIG. 1 a  is a schematic illustration of prior art recession caused by CMP. 
     FIG. 1 b  is a schematic illustration of prior art erosion caused by CMP. 
     FIG. 2 is a graph of the frequency dependence of β eff . 
     FIG. 3 is a side view of an ER fluid between a metal wafer surface and a polishing pad, with an applied electrical field therebetween. 
     FIG. 4 illustrates an embodiment of the invention wherein electrical contact is made to the wafer edge. 
     FIG. 5 a  illustrates an embodiment of the invention wherein electrical contact is made to the wafer front. 
     FIG. 5 b  illustrates an embodiment of the invention wherein a high electric field region is established near the polishing pad. 
     FIG. 6 illustrates another embodiment of the invention wherein electrical contact is made to the wafer front. 
     FIG. 7 illustrates an embodiment of the invention wherein electrical contact is made to the wafer back side. 
     FIG. 8 illustrates the relationship between electrical field frequency and metal pattern density. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 a  illustrates the prior art problem of recession seen with metal CMP. Metal  2  between oxide regions  4  is removed at a higher rate than the oxide, thereby causing a local recessed region  6  to be formed. FIG. 1 b  illustrates the problem of erosion, also seen with metal CMP. Erosion is a global effect rather than a local effect as in recession. In the example shown in FIG. 1 b , the center portion  8  of the wafer is polished at a higher rate than the outer portion  10 , thereby leaving a more highly eroded region  12 . 
     According to a first set of embodiments of my invention, an electrorheological (ER) fluid is used as a component of a CMP slurry for metal layers in Damascene processing, with an external electric field applied across the ER fluid. The shear strength, or yield stress τ 0  of the ER fluid is defined as the limit of the shear stress of the fluid as the shear rate approaches zero. The yield stress depends on both the electric field strength and on the frequency of the applied field according to the Maxwell-Wagner electrostatic polarization model, as 
     
       
         τ 0 (E,ω)∝E 2 (ω)  (1) 
       
     
     where 
     τ 0 (E,ω)=yield stress, 
     E=alternating electric field strength 
     ω=frequency of the alternating electric field, 
     β eff =effective relative particle polarizability 
     The Maxwell-Wagner model predicts that the yield stress will increase with the square of the externally applied electric field. Additionally, the model predicts that the yield stress will increase with the square of the effective polarizability. A description of the electrostatic polarization mechanism is given by W. M. Winslow in J. Appl. Phys. 20, 1949, p 1137, which is hereby incorporated by reference. 
     β eff depends on dielectric constants and conductivities of particulate and continuous phases of the ER fluid in question, as well as on frequency of the applied field. For particulates with relatively large dielectric constants, β eff   2  increases with frequency, whereas for particulates with relatively small dielectric constants, β eff   2  decreases with frequency, as illustrated in FIG.  2 . 
     It can be shown that the viscosity η varies linearly with τ 0 (E) for a given shear rate. It is believed that, when using an ER fluid, the metal polish rate will vary directly as the viscosity of the fluid if all other factors such as rotational velocity, downward force on the spindle, etc., are held constant. It is believed that, according to the local topography of the metal surface being polished, the electric field strength will vary across the ER fluid, and thus will automatically tune the CMP polish rate, as described hereinafter. 
     FIG. 3 is a schematic diagram showing a wafer  20  having an oxide layer  21  and a metal layer  22  thereon, a platen  41  with polishing pad  50  thereon, and an ER fluid  26  therebetween. The ER continuous phase material must have insulating characteristics. A voltage V is applied between metal layer  22  and platen  41 . An electric field is thereby established across ER fluid  26 . In the first approximation of a uniform electric field, the relationship between voltage difference V and electric field E is expressed as: 
     
       
         E=V/d  (5) 
       
     
     where d is the distance between the two points having voltage difference V therebetween. 
     It is seen, therefore, that in the uniform field approximation, the electric field between metal layer  22  and platen  41  varies inversely with the distance therebetween. For raised metal regions  28 , the local electric field  30  across ER fluid  26  is greater than the local electrical field  32  across ER fluid  26  for lower metal regions  34 . According to equations (1), (2), and (3), a higher electric field E across the ER fluid results in a higher yield stress τ 0  and higher viscosity η of the ER fluid. Higher viscosity of the ER fluid in regions of higher E during polishing will cause the polishing rate to be higher in regions of higher electric field E. Accordingly, raised metal regions  28  will polish at a higher rate than lower metal regions  34 , resulting in an automatic tuning of the polish rate to achieve increased flatness of the metal surface. Since the metal layer  22  is used as one of the electrodes, a built-in and localized polishing endpoint occurs as the metal electrode is polished away and the local electric field across the gap goes to zero. 
     In order to achieve the aforementioned chemical and abrasive components of CMP, a chemical oxidizer and abrasive may be incorporated into the ER continuous phase material. A requirement of any such combination of chemical oxidizer, abrasive, and ER material is its insulating characteristics. 
     The voltage V may be applied across the ER fluid in many different ways, depending on the configuration and structure of the wafers being polished, as well as the details of the polisher. FIG. 4 illustrates a first embodiment wherein one or more electrical contacts  36  are placed on edge  38  of wafer  20 . Voltage V may be applied between contacts  36  and platen  41 . (In this and all other embodiments described herein, platen  41  may be stationary, rotatable, or linearly movable). This embodiment can be used for a metallization process which provides metal deposited or electroplated on the wafer edge  38  as well as on the wafer top surface  44 , whereby voltage V applied to contacts  36  is simultaneously applied to metal layer  22  which is contiguous with contacts  36 . Contacts  36  may be, by way of example, pins surrounding the edge of the wafer which have thickness less than the thickness of the wafer. 
     FIG. 5 a  illustrates a second embodiment of my invention which provides electrical contact with the front side of the wafer having the metal layer  22  thereon. Wafer  20  is held against spindle  40  by vacuum suction, and is brought into contact with, and moved relative to (generally rotated against), polishing pad  50  having ER fluid  52  thereon. Polishing pad  50  is made of an insulating material; atop metal electrode  54  which is positioned on insulating platen  41 . Electrical contact  58  is made to electrode  54 . Soft conducting contacts  60  are embedded in an array across polishing pad  50 , and protrude from its surface. Contacts  60  are aligned against metal pins  62  which are directed through platen  41  and through insulating sheaths  64  embedded in metal electrode  54 . Voltage V is applied between electrode  54  and contacts  60 . As wafer  20  is brought into close proximity to polishing pad  50 , physical contact is made between some or all of contacts  60  and metal layer  22  atop wafer  20 . Voltage V is. thereby established between metal layer  22  and electrode  54 , across ER fluid  52 . Electric field lines  65  show that the electric field is higher at some raised metal regions  28  than at lower metal regions  34 , thereby providing stiffer ER fluid and higher abrasion rate at some raised regions  28 . 
     The embodiment shown in FIG. 5 a  can be modified slightly, as shown in FIG. 5 b , to provide a different type of electrical field configuration. If contacts  60  are designed to protrude only slightly or not at all from the surface of polishing pad  50 , so as not to come into physical contact with metal layer  22 , the electric field lines  65 ′ will be configured as shown in FIG. 5 b , with a high field region  67  extending into the region between contact  60  and metal layer  22 . Accordingly, the ER fluid in region  67  will stiffen. Raised metal regions  28  pass through this stiffened fluid region and are abraded more aggressively than lower metal regions  34  which do not encounter the stiffened ER fluid. Once all the high points on metal layer  22  have been removed, the polish rate will be even across the metal layer. As soon as any part of the metal layer  22  physically contacts contact  60  and thus causes entire metal layer  22  to be at voltage V, the electric field in the ER fluid in region  67 , and therefore the stiffness of the ER fluid, will be significantly reduced because the effective electrode size will be significantly increased. It is well known that the electric field near a sharp edge electrode is much greater than near an electrode with a large surface area. An aspect of this embodiment, which may be advantageous or disadvantageous according to the particular application, is that the stiffened region  67  of the ER fluid is stationary near contacts  60 , and is not subject to frequency dependent effects as described hereinafter. Applied voltage V may be either DC, yielding a constant stiffened ER fluid region  67 , or AC, wherein the stiffened region  67  would occur periodically. 
     Other methods of establishing electrical contact with the front side of the wafer having the metal layer thereon might be utilized. By way of example, a conduit might be bored through the wafer at a chosen location, and a wire could be inserted therethrough from the wafer back side to make contact with the metal layer. The voltage would then be applied from the back side of the wafer. 
     FIG. 6 illustrates a third embodiment of my invention wherein electrical contact is made to the front side of the wafer. Notch  66  is etched into outer region  68  of wafer  20 , and metal contact  70  is deposited into at least a portion of notch  66  simultaneously with deposition of metal layer  22 . If metal is not deposited all the way to edge  38  of the wafer, a conducting lead  71  may be embedded in notch  66  and electrically connected to contact  70 , without protruding above the wafer surface, and extending beyond edge  38 . Metal contact  70  is contiguous with metal layer  22 , and therefore when voltage V is applied between lead  71  and platen  41 , it is simultaneously applied between metal layer  22  and platen  41 . 
     FIG. 7 illustrates a fourth embodiment of my invention wherein electrical contact is made to the back side of the wafer. Voltage V is applied between platen  41  and contact  72  on backside  74  of wafer  20 . It is known, for example in using defect inspection tools, that electrical contact with the backside of the wafer provides sufficient conduction through the wafer to establish a voltage on the wafer surface, for low current applications such as that of the present invention. This approach avoids the possibility of leaving islands of the metal on the surface of the wafer with no ground during processing, but introduces the possibility of charging formed devices by driving currents through the wafer. 
     Another aspect of my invention relates to the frequency dependence of the yield stress and therefore the viscosity of the ER fluid, according to equation (1): 
     
       
         τ 0 (E,ω)∝E 2 β eff   2 (ω)  (1) 
       
     
     The frequency dependence of the yield stress τ 0  stems from the frequency dependence of the effective relative particle polarizability β eff , a quantity which relates to the conductivities and dielectric constants of the particulate and continuous phase of the ER material. This is described more fully by Klingenberg in the MRS Bulletin, August 1998, pp 30-34, which is hereby incorporated by reference. The ER fluid and its components can be chosen so as to exhibit a desired frequency dependence. For example, suspensions of barium titanate particles in silicon oil evidence a yield stress which increases with frequency. In contrast, for suspensions of alumina particles in silicone oil, the yield stress decreases with frequency. 
     If the voltage is applied across the ER fluid  26  by establishing a voltage difference between platen  41  and metal layer  22 , and assuming the voltage is DC, the frequency (ω) of the electric field across the ER fluid  26  depends on two factors: (1) the rotational velocity of the spindle  48  and wafer  20  relative to the polishing pad  50 , and (2) the number of metal lines in a given area, i.e., the metal pattern density. The higher the pattern density, the higher the electric field frequency across the ER fluid at a given point on the stationary platen  41 . This effect is illustrated in FIG. 8, assuming a stationary ER fluid for simplicity. 
     By tailoring the ER fluid to the application, the frequency dependence of the yield stress can be advantageously utilized. For example, in standard CMP of Damascene structures, it is well known that erosion is greater in the high line density areas. Using my polishing approach, by choosing an ER fluid which evidences lower yield stress for higher frequency, such as the aforementioned alumina in silicone oil, the polishing rate would be lower in the high density areas and thereby the cause of the erosion problem would be ameliorated. 
     As feature sizes decrease and line density increases, the ER fluid response time must also decrease. For example, with a metal feature pitch of 0.32 microns, and with typical spindle rotational velocities, the electric field frequency imposed on an ER fluid would exceed 10 MHz. The response time of the dielectric dispersion in the ER fluid preferably will equal or exceed this frequency. 
     Another embodiment of my invention utilizes a MR fluid in place of, or in combination with, an ER fluid for metal CMP. The electric field is a local quantity which varies according to localized topography and feature density, and therefore use of ER fluid provides localized tuning of polishing rates. In contrast, use of MR fluids allows global adjustments of polishing rates. For example, if the polishing rate is greater at the center of the wafer, as illustrated in FIG. 1 b , an external magnetic field can be established which has higher magnitude at the wafer edge region, in order to equalize the polishing rate across the wafer. 
     A composite of ER particles and MR particles dispersed in a continuous phase increases the complexity of the slurry and the process, yet allows for even more control over the polishing process. As previously described, the ER fluid can locally tune the polishing rates, while the MR fluid can establish polishing zones to improve the global planarity of the wafer surface. 
     By utilizing ER and MR fluids as components of polishing slurries according to my invention, many of the problems currently existing with CMP of metal can be avoided, including recession, erosion, and other types of non-uniformity of polish rate across the wafer. 
     The invention should not be considered limited to the exact implementations and embodiments described herein. For example, other methods of providing electrical contact to the metal layer on the wafer, or to establish an electric field across the ER fluid, may be utilized without altering the inventive concept. The scope of the invention should be construed in view of the claims.