Abstract:
A pedestal cover for a semiconductor wafer. The wafer is positioned overlying the pedestal cover in a material deposition chamber, with the cover defining a peripheral circumferential trench therein. During the material deposition process, deposited material is formed within the trench and the build up of material adjacent a peripheral edge of the wafer is thereby avoided.

Description:
[0001]    This application claims the benefit of provisional patent application Serial No. 60/470,120 filed on May 13, 2003. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates generally to the formation of aluminum metallization layers for an integrated circuit device, and more specifically to a wafer pedestal cover for use during the formation of an aluminum metallization layer on a wafer.  
         BACKGROUND OF THE INVENTION  
         [0003]    Integrated circuit devices (or chips) typically comprise a silicon substrate and semiconductor elements, such as transistors, formed from doped regions within the substrate. Interconnect structures, formed in parallel layers overlying the semiconductor substrate, provide electrical connection between semiconductor elements to form electrical circuits. Typically, several (e.g., 6-9) interconnect layers (each referred to as an “M” or metallization layer) are required to interconnect the doped regions and elements in an integrated circuit device. The top metallization layer provides attachment points for conductive interconnects (e.g., bond wires) that connect the device circuit&#39;s off-chip, such as to pins or leads of a package structure.  
           [0004]    Each interconnect structure comprises a plurality of substantially horizontal conductive interconnect lines or leads and a plurality of conductive vertical vias or plugs. The first or lowest level of conductive vias interconnects an underlying semiconductor element to an overlying interconnect line. Upper level vias connect an underlying and an overlying interconnect line. The interconnect structures are formed by employing conventional metal deposition, photolithographic masking, patterning and etching techniques. One material conventionally used for the horizontal conductive interconnect layers comprises aluminum. To form the interconnect lines the aluminum is blanket deposited over an intermetallic dielectric layer disposed on an upper surface of the substrate, then patterned according to conventional techniques to form the desired interconnect lines. The material of the conductive vias conventionally comprises tungsten.  
           [0005]    Sputtering, also known as physical vapor deposition (PVD), is one known technique for blanket depositing aluminum on the intermetallic dielectric layer. One example of a prior art sputtering process chamber  100  is illustrated in FIG. 1, in which the components are illustrated in the wafer load position, i.e., when the wafer is loaded into the chamber. The chamber  100 , which is maintained at a vacuum during the deposition process, encloses a target  102  formed from a material to be deposited on a wafer  106  located near the bottom of the chamber  100 . The target  102  is negatively biased with respect to a chamber shield  108  (which is typically grounded) by a direct current power supply  110 . Conventionally, argon molecules are introduced into the chamber  100  via an inlet  112  and ionized by the electric field between the target  102  and the chamber shield  108  (i.e., ground) to produce a plasma of positively charged argon ions  116 . The argon ions  116  gain momentum as they accelerate toward the negatively charged target  102 .  
           [0006]    A magnet  118  creates a magnetic field that generally confines the argon plasma to a region  117 , where the increased plasma density improves the sputtering efficiency. As the argon ions  116  bombard the target  102 , the momentum of the ions is transferred to the molecules or atoms of the target material, sputtering or knocking these molecules or atoms from the target  102 . A high density of argon ions  116  in the chamber  100  ensures that a significant number of the sputtered atoms condense on an upper surface of the wafer  106 . The target material, in the case of aluminum, is deposited on the wafer  106  without undergoing any chemical or compositional changes. The various sputtering process parameters, including chamber pressure, temperature and deposition power (i.e., the amount of power (the product of voltage and current) supplied to the target  102  by the power supply  110 ) can be varied to achieve the desired characteristics in the sputtered film. Generally, a higher target power increases the target deposition rate.  
           [0007]    Prior to initiating the deposition process, a robot arm (not shown in FIG. 1) transports the wafer  106  into the chamber  100  and positions the wafer  106  on a plurality of wafer lift pins  124 . As a chuck  126  is driven upwardly, retracting the pins  124  into the chuck  126 , the wafer  106  comes to rest on pads  127  of a pedestal cover  128  overlying an upper surface  129  of the chuck  126 .  
           [0008]    As the chuck  126  continues moving upwardly, the wafer  106  contacts a clamp assembly  130  (a ring-like structure) supported by a wafer/clamp alignment tube assembly  132 . The chuck  126  continues the upward motion until the clamp  130 , the wafer  106 , and the chuck  126  are in the process position illustrated in FIG. 2. The deposition process is then initiated. During the sputtering process the force exerted between the clamp and the chuck  126  holds the wafer  106  in place against the pads  127 . This final process position is referred to as the source to substrate spacing, where the target  102  is the source and the wafer  106  is the substrate. The spacing is determined to provide the optimum deposition uniformity during the sputtering process.  
           [0009]    When the deposition process has ended, the above steps are executed in reverse order to remove the wafer  106  from the chamber  100 . The robot arm transfers the wafer to the next chamber for execution of the next process step.  
           [0010]    As is known, the clamp  130  is a ring-like structure that contacts only the wafer periphery. In one embodiment, the wafer diameter is about 200 mm with a peripheral edge exclusion area  140  (see FIG. 3) of about 3 mm in which no semiconductor devices are fabricated. The clamp  130  contacts the wafer  106  at a contact point  141  within about 1 mm of the wafer bevel edge  142 . However, a clamp region  143  extending beyond the contact point  141  shadows the wafer  106 . Thus the edge exclusion area  140  comprises a peripheral ring region about 3 mm wide, which reduces the active wafer area.  
           [0011]    During aluminum sputtering on the surface of the wafer  106 , an aluminum deposit  144  is formed on an upper surface  145  of the clamp  130 , producing an additional shadowing effect on the wafer  106 . This shadowing effect can extend beyond the 3 mm edge exclusion area  140 .  
           [0012]    As the deposition of aluminum on the upper surface  145  continues during deposition processing in the chamber  108 , eventually the aluminum deposit  144  can contact an upper surface  146  of the wafer  106  at a contact point  147  as illustrated in FIG. 4. At the contact point  147  a weld-like effect is created between the wafer  106  and the clamp  130 . When this occurs, the wafer  106  may not be separable from the clamp  130  after the aluminum deposition process is completed.  
           [0013]    Use of the clamp  130  can also cause the formation of defect particulates on the wafer  106 . Returning to FIG. 1, the wafer/clamp alignment tube assembly  132  is adjustable to align the clamp  130  relative to the wafer  106 . But the metal-to-metal contact between the clamp  130  and the wafer/clamp alignment tube assembly  132  is a generating source for particles that can fall onto the upper surface  146 , creating potential wafer defects and reducing the process yield.  
           [0014]    An electrostatic chuck is known to overcome certain disadvantages associated with use of the clamp  130 . An electrostatic chuck holds the wafer  106  in a stable, spaced-apart position by an electrostatic force generated by an electric field formed between the wafer  106  and the chuck It is known, however, that this electric field can detrimentally affect the material deposition process by generating backside particles during the de-chucking process, i.e., removing the wafer  106  from the chamber  100 . There is also a measurable thermal gradient across the electrostatic chuck, resulting in aluminum grain variations across the wafer  106 . In particular, increased levels of backside particles and changes in the grain orientation have been observed, especially near the wafer center. Electrostatic chucks are considerably more expensive than the wafer clamp system and have a shorter useful life.  
           [0015]    In both the clamped and electrostatic chucks, embedded heaters heat the chuck to a predetermined temperature (e.g., about 300° C.) to maintain a desired wafer temperature. In both chuck types, a gas (usually argon) flows behind the wafer  106  to thermally couple the chuck  126  and the wafer  106  to maintain the wafer temperature at the chuck temperature. The gas is introduced to the wafer backside through an orifice  149  in the chuck  126 . See FIGS. 1 and 2. Since the frictional forces of the impinging sputtered atoms can raise the wafer temperature above the chuck temperature, the gas (referred to as backside cooling) cools the wafer  106  as it flows between the wafer  106  and the chuck  126 . With heat transfer from the gas, the chuck may also serve as a heat sink The backside cooling gas is withdrawn from the chamber  108  by a cryogenic pump (not shown in the Figures) operable to maintain the chamber vacuum. If the backside cooling gas is not evenly distributed across the wafer bottom surface, hot spots and attendant aluminum defects can appear in the deposited layer. It has been observed that without backside cooling the wafer temperature increases with time, approaching the plasma temperature. Such excessive wafer temperatures can cause defects in the deposited aluminum and also destroy the wafer. Thus it is known that controlling the chuck temperature during the deposition process, together with the use of backside cooling (and a clamp in the clamp-type chucks) provides control over the wafer temperature to improve the material deposition process.  
           [0016]    Electromigration is a known problem for aluminum interconnect leads in integrated circuit devices. The current carried by the long, thin aluminum leads produces an electric field in the lead that decreases in magnitude from the input side to the output side. Also, heat generated by current flow within the lead establishes a thermal gradient. The aluminum atoms in the conductor become mobile and diffuse within the conductor in the direction of the two gradients. The first observed effect is conductor thinning, and in the extreme case the conductor develops an open circuit, causing the device to malfunction.  
           [0017]    It is known that use of aluminum alloys, including alloys of copper, silicon and aluminum, can reduce electromigration effects. However, these aluminum alloys present increased complexity for the deposition equipment and processes, and exhibit different etch rates than pure aluminum, necessitating process modifications to achieve the desired etch results. Compared with pure aluminum, the alloys may exhibit increased film resistivity and thus increased lead resistance.  
           [0018]    The interconnect leads in an integrated circuit device are also under considerable mechanical stress due to thermally induced expansion and contraction during operation. These effects contribute to stress voiding failure mechanisms in which the interconnect metal separates, creating a void.  
           [0019]    It has been shown that the aluminum grain orientation and grain size affect the electromigration and stress voiding characteristics of an aluminum interconnect lead. In particular, an aluminum grain orientation along the &lt;111&gt; plane is known to produce minimal electromigration effects. According to the prior art, when aluminum is deposited over a titanium/titanium nitride stack, which is a typical stack composition, the aluminum grain orientation is controlled by the underlying titanium orientation. The titanium-nitride orientation is also controlled by the titanium orientation. Thus if the titanium orientation is correct (i.e., a Miller index of &lt;002&gt; the overlying aluminum will have a high probability of exhibiting a &lt;111&gt; orientation. According to the prior art, the wafer temperature affects the aluminum grain size and the grain orientation.  
         BRIEF SUMMARY OF THE INVENTION  
         [0020]    The present invention teaches a physical vapor deposition chamber for depositing material on a wafer. The chamber comprises a chuck for supporting the wafer during the deposition process and a pedestal cover overlying an upper surface of the chuck and extending beyond sidewalls of the chuck The wafer is positionable over the pedestal cover. The pedestal cover defines a peripheral circumferential groove therein. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    The foregoing and other features of the present invention will be apparent from the following more particular description of the invention as illustrated in the accompanying drawings, in which like reference characters refer to the same parts throughout the different figures. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.  
         [0022]    [0022]FIGS. 1 and 2 illustrate prior art physical vapor deposition chambers.  
         [0023]    [0023]FIGS. 3 and 4 illustrate the contact between prior art wafer clamps and the wafer.  
         [0024]    [0024]FIGS. 5 and 6 illustrate a physical vapor deposition chamber according to the teachings of one embodiment of the present invention.  
         [0025]    [0025]FIG. 7 is a close-up cross-sectional view of a prior art pedestal cover.  
         [0026]    [0026]FIG. 8 is a close-up cross-sectional view of a pedestal cover constructed according to the present invention.  
         [0027]    [0027]FIG. 9 is a top view of a wafer and a pedestal cover constructed according to the teachings of the present invention.  
         [0028]    [0028]FIG. 10 is a close-up cross-sectional view of a pedestal cover constructed according to another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]    Before describing in detail the particular pedestal cover in accordance with the present invention, it should be observed that the present invention resides in a novel and non-obvious combination of elements. Accordingly, these elements have been represented by conventional elements in the drawings, showing only those specific details that are pertinent to the present invention so as not to obscure the disclosure with details that will be readily apparent to those skilled in the art having the benefit of the description herein.  
         [0030]    [0030]FIGS. 5 and 6 illustrate a clampless chuck  150  for use in a physical vapor deposition chamber as described and claimed in a commonly-owned patent application entitled, Apparatus and Method for Producing a &lt;111&gt; Orientation Aluminum Film for an Integrated Circuit Device, filed on Jul. 8, 2003, and assigned application Ser. No. 10/615,583. In FIG. 5 the elements are illustrated in the wafer load position. FIG. 6 illustrates the same elements in the deposition process position.  
         [0031]    The wafer weight exerts a downwardly directed force that holds the wafer  106  against the pads  127  of the pedestal cover  128 . Wafer backside cooling is not required. Thus absent backside cooling, there is no coolant fluid force directed against the bottom surface of the wafer  106  and no need for an additional downward force, such as by use of a clamp, to overcome the coolant fluid force. Advantageously, avoiding use of a clamp permits semiconductor devices to be fabricated in the wafer edge exclusion area  140  that is obscured by the prior art clamp  130 .  
         [0032]    It has been determined that the wafer temperature affects both aluminum grain size and grain orientation. The underlying material layer should be in a predetermined orientation so that the sputtered aluminum grows in the preferred orientation. Although the influence of wafer temperature on grain orientation may not be as significant as the orientation of the underlying layer (titanium for example), the number of aluminum atoms exhibiting a &lt;111&gt; crystal orientation increases when the wafer is maintained within a predetermined temperature range. Maintaining the desired wafer temperature provides the thermal characteristics required for proper growth of the aluminum material layer. If the thermal properties of the deposition are not properly maintained, alloys of the target material precipitate to the aluminum grain boundaries, which will have a detrimental effect on the aluminum film growth. Such alterations in the aluminum film directly impact the orientation of the aluminum atoms.  
         [0033]    It has further been determined that a wafer temperature of between about 245° C. and 285° C. produces an advantageous aluminum grain size (about 0.8 microns) with a substantial majority of the grains in the &lt;111&gt; crystal plane. According to the teachings of the present invention, the chuck temperature is controlled to achieve a wafer temperature in this range, taking into consideration the various chamber and process parameters that affect the chuck temperature, the wafer temperature, and the functional dependence between the wafer temperature and the chuck temperature.  
         [0034]    To control the wafer temperature, the various uncontrolled process effects that influence the wafer temperature should be minimized. In the FIG. 6 configuration the wafer  106  is spaced apart from the target  102  such that at a distance of about 45 mm, the heat generated by the plasma and by the frictional forces of the impinging deposition particles are not dominant heat sources for the wafer  106 . Instead, the wafer temperature is determined primarily by radiant heat flow from the chuck  150 , as heated by chuck heaters  156  under control of a temperature controller  158 . Because the wafer  106  is not in direct physical contact with the chuck  126 , being separated therefrom by the height of the pads  127  on the pedestal cover  128  (typically, the pads  127  are about 2 mm in height) there is minimal conductive heat flow between the wafer  106  and the chuck  150 .  
         [0035]    It has been determined that a chuck temperature of between about 350° C. and 450° C. produces a wafer temperature of between about 245° C. and 285° C. At a chuck temperature of about 450° C. the wafer temperature of the present clampless process matches the temperature of the wafer in the prior art clamp processes, and the properties of the deposited film are substantially similar to those observed with the clamped chuck  
         [0036]    Although the chuck temperature is determined primarily by the controllable chuck heaters  156 , the heat transfer between the chuck  126  and the wafer  106  is also influenced by certain characteristics of the PVD chamber  100 . For example, the heat flow from the chuck  126  to the wafer  106  depends on the distance between the wafer  106  and the upper surface  129  of the chuck  126 , i.e., the height of the pads  127  on the pedestal cover  128 . The wafer temperature also depends on the duration of the deposition process, i.e., the time that the wafer  106  is subjected to the high-temperature deposition plasma and the frictional forces of the sputtered particles.  
         [0037]    Additionally, in one embodiment the wafer temperature upon entering the PVD chamber  100  can be measured (using an optical pyrometer in one embodiment) and considered in establishing the chuck temperature. The entry temperature is dependent on the previous processes to which the wafer had been subjected, and the time required to transfer the wafer  106  from the previous chamber to the chamber  100 . It is known that in certain processing tools the wafer temperature drops about 0.5° C./second while the wafer moves between tool chambers. Thus in one embodiment the chuck temperature, as controlled by the temperature controller  158 , is also responsive to the initial wafer temperature, such that a wafer temperature of about 285° C. is maintained during the PVD process of the present invention.  
         [0038]    In yet another embodiment, the wafer temperature is determined during the deposition process and the temperature value feedback to the temperature controller  158  for controlling the chuck heaters  156  in response thereto.  
         [0039]    [0039]FIG. 7 is a close-up cross-sectional view of a portion of the wafer  106 , the chuck  126 , the pads  127  and the pedestal cover  128  of the prior art. The pads  127 , which are elements of the pedestal cover  128 , hold the wafer  106  about 2 mm above a top surface  201  of the pedestal cover  128 . As shown, the pedestal cover  128  overlies the chuck  126 . Pedestal covers are conventionally used in PVD tools to avoid depositing material onto the chuck in the event the tool is activated without a wafer in the deposition position. Thus the pedestal cover can be easily removed and replaced by a new pedestal cover in the event material is mistakenly deposited on the pedestal cover. Replacement of a damaged pedestal cover is considerably simpler and less expensive than replacing the chuck  126 . During the deposition process, the pedestal cover  128  also serves to shield chamber components located generally in an area  202  surrounding the chuck  126 .  
         [0040]    As can be seen in FIG. 7, absent a clamp overlying and exerting a downward force on the wafer  106  (as depicted in FIGS. 5 and 6) during the deposition process an aluminum mass  208  forms on a peripheral surface  210  of the prior art pedestal cover  128 . The aluminum mass  208  is deposited during the physical vapor deposition process and continues to grow during each subsequent deposition in the chamber  100 . If the aluminum mass  208  extends over an upper surface  212  of the wafer  106  (for example, at an edge  214 ), when the lift pins  124  are raised to remove the wafer  106  from the chamber  100 , the wafer  106  is not free to be lifted from the pedestal cover  128 . Instead, the wafer  106  can be cracked as the lift pins apply an upwardly directed force to the wafer  106 , and contact between the upper surface  212  and the edge  214  exerts a downwardly directed force on the wafer  106 .  
         [0041]    According to an embodiment of the present invention, a pedestal cover  220  comprises a trench  222  around a peripheral edge region  224  thereof. See the cross-sectional view in FIG. 8. In one embodiment the trench  222  is formed by milling material from a peripheral edge region  224 . The mass differential between the prior art pedestal cover  128  and the pedestal cover  220  of the present invention has been determined not to affect the thermal budget of the chamber  200  so as to necessitate a change in other process parameters. The pedestal cover  220  exhibits a considerably longer life than the prior art pedestal cover  200 , as the processing time required to deposit an interfering aluminum mass  208  is much longer.  
         [0042]    [0042]FIG. 9 illustrates a top view of the pedestal cover  220 , the trench  222  and the wafer  106 .  
         [0043]    In another embodiment, illustrated in FIG. 10, a pedestal cover  230  is disposed above and in contact with the chuck  126 . The pedestal cover  230  comprises a support member  231  and a sidewall  232  extending downwardly from the support member  231 . The sidewall  232  does not extend beyond the edge  214  of the wafer  106 . Thus the pedestal cover  230  does not present an exposed surface on which aluminum can accumulate during the deposition process.  
         [0044]    Suppliers of deposition chambers and vendors of related equipment provide a service whereby certain chamber parts, collectively referred to as a kit, are cleaned after a period of use (about 800 kilowatt-hours) in the deposition chamber. One such kit comprises various replaceable chamber parts that shield non-replaceable chamber parts during the deposition process. The pedestal cover is one component of the shielding parts kit. The kit parts are removed from the deposition chamber and sent to the vendor, where they are cleaned using acid baths, and other known methods, to remove the aluminum from the stainless steel kit parts. A pedestal cover according to the teachings of the present invention, for example as depicted in FIGS.  8  or  10 , can be included within a shielding parts kit. With the trench  222  in the FIG. 8 embodiment, the time interval between cleanings for the pedestal cover  220  can be as long as four times the cleaning interval for prior art pedestal covers.  
         [0045]    While the invention has been described with reference to preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalent elements may be substituted for elements thereof without departing from the scope of the present invention. The scope of the present invention further includes any combination of the elements from the various embodiments set forth herein. In addition, modifications may be made to adapt a particular situation to the teachings of the present invention without departing from its essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.