Method and apparatus for chemical polishing using field responsive materials

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.

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. 1a
 and 1b. 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.

DETAILED DESCRIPTION OF THE INVENTION
 FIG. 1a 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. 1b
 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. 1b, 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
 .tau..sub.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
EQU .tau..sub.0 (E,.omega.).varies.E.sup.2 (.omega.) (1)
 where
 .tau..sub.0 (E,.omega.)=yield stress,
 E=alternating electric field strength
 .omega.=frequency of the alternating electric field,
 .beta..sub.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.
 .beta..sub.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, .beta..sub.eff.sup.2 increases with frequency,
 whereas for particulates with relatively small dielectric constants,
 .beta..sub.eff.sup.2 decreases with frequency, as illustrated in FIG. 2.
 It can be shown that the viscosity .eta. varies linearly with .tau..sub.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:
EQU 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 .tau..sub.0 and higher viscosity .eta. 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. 5a 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. 5a can be modified slightly, as shown in FIG.
 5b, 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. 5b, 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):
EQU .tau..sub.0 (E,.omega.).varies.E.sup.2.beta..sub.eff.sup.2 (.omega.) (1)
 The frequency dependence of the yield stress .tau..sub.0 stems from the
 frequency dependence of the effective relative particle polarizability
 .beta..sub.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 (.omega.) 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. 1b, 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.