Abstract:
Correction of skew in plasma etch rate distribution is performed by tilting the overhead RF source power applicator about a tilt axis whose angle is determined from skew in processing data. Complete freedom of movement is provided by incorporating exactly three axial motion servos supporting a floating plate from which the overhead RF source power applicator is suspended.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 12/787,193, filed May 25, 2010 entitled PLASMA REACTOR WITH TILTABLE OVERHEAD RF INDUCTIVE SOURCE, by Kenneth Collins, et al., which claims the benefit of U.S. Provisional Application Ser. No. 61/239,711, filed Sep. 3, 2009 entitled PLASMA REACTOR WITH TILTABLE OVERHEAD RF INDUCTIVE SOURCE, by Kenneth Collins, et al. 
    
    
     BACKGROUND 
     Plasma etch processes are employed in microelectronic circuit fabrication to define thin film structures on semiconductor wafers or workpieces. Generally, a disc-shaped workpiece is processed in a cylindrical reactor chamber. Features sizes in the thin film structures formed, by the etch process can be as small as tens of nanometers, for example. Uniformity of etch rate distribution across the entire surface of the workpiece is critical In attaining workable devices. The etch rate distribution reflects the plasma ion density distribution across the workpiece surface existing during the plasma etch processing of the workpiece. Etch processes can employ an inductively coupled RF plasma in which the plasma source consists of a coil antenna over the ceiling of the reactor chamber. The etch rate distribution can have a radial non-uniformity, in which the non-uniformity pattern is generally symmetrical about the cylindrical axis of symmetry of the reactor chamber. For example, the etch rate distribution may reflect a plasma ion density distribution that is, predominantly, either center-high or center low. Such a radial pattern of non-uniformity can be corrected by dividing the ceiling coil antenna into two or more concentric coil antennas that are separate from one another and are independently powered with RF power. Radial non-uniformity in etch is corrected in such a reactor by adjusting the RF power levels independently delivered to the separate concentric coil antennas. While this arrangement works well in correcting radial non-uniformities in etch rate distribution, it is not well-suited for correcting for asymmetrical non-uniformities in etch rate distribution. Such asymmetrical non-uniformities may be referred to as “skew” non-uniformities, and typically are manifested as a difference between etch rates on opposite sides of the workpiece. As one simplified example, one half of the workpiece may experience a higher etch rate than the other half. Under real production conditions, it is often found that the etch rate distribution measured across the surface of the workpiece has both radial non-uniformity and skew non-uniformity in combination. If the skew non-uniformity could be somehow corrected or eliminated, then the remaining non-uniformity, namely the radial non-uniformity, could foe eliminated, by apportioning the RF power levels delivered to the different concentric overhead coil antennas. The result would be correction of all etch rate distribution non-uniformity across the workpiece surface. The problem is how to eliminate the skew non-uniformity in etch rate distribution. 
     SUMMARY 
     A plasma reactor for processing a workpiece includes a processing chamber enclosure defining a process chamber interior and comprising a chamber side wall and a chamber ceiling, and a workpiece holder inside the process chamber interior and a conductive RF enclosure overlying the ceiling and comprising an RF enclosure side wall and an RF enclosure top cover. A shoulder ring is supported on the RF enclosure side wall, and a floating support plate is placed inside the conductive RF enclosure and adjacent the shoulder ring. Plural radially inner and outer RF plasma source power applicators are suspended from the floating support plate in a space below the floating support plate and above the chamber ceiling. Plural RF power sources are coupled to a corresponding one of the plural RF plasma power applicators. Plural actuators fixed with respect to the shoulder ring are spaced about the shoulder ring at periodic intervals. Each one of the plural actuators has an axially movable arm and a motor driving the movable arm in an axial direction. A rotatable joint having two joint ends is provided, one of the joint ends being connected to the axially movable arm and the other of the joint ends being connected, to a portion of the floating support plate adjacent the one actuator, whereby the floating plate is supported at respective plural locations by the rotatable joint of each respective one of the plural actuators. Only three actuators are provided in the preferred embodiment, to ensure complete freedom of movement of the floating support plate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the exemplary embodiments of the present invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings. It is to be appreciated that certain well known processes are not discussed herein in order to not obscure the invention. 
         FIG. 1  is a partially cut-away side view of a reactor in accordance with an embodiment. 
         FIG. 2  is a top view corresponding to  FIG. 1 . 
         FIG. 3  is an enlarged view of a portion of the reactor of  FIG. 1 . 
         FIG. 4  is another enlarged view corresponding to  FIG. 3 . 
         FIG. 5  is a block diagram of a control system included in the reactor of  FIG. 1 . 
         FIG. 6  is a block flow diagram depicting operation of the control system of  FIG. 5 . 
         FIG. 7  depicts a coordinate system employed in the control system of  FIG. 5  to control the motion of the overhead coil source of  FIG. 1  in accordance with an embodiment. 
         FIG. 8  is a diagram depicting the three-dimensional locations of the actuators of the reactor of  FIG. 1  in one implementation. 
         FIG. 9  depicts a reactor in accordance with alternative embodiment. 
     
    
    
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation. It is to be noted, however, that the appended drawings illustrate only exemplary embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
     DETAILED DESCRIPTION 
     Referring  FIGS. 1-4  depict a reactor of the type employed to carry out a reactive ion etch process using an RF inductively coupled plasma. The reactor includes a chamber enclosure  10  including a disk-shaped ceiling  12  and a cylindrical side wall  14  defining a chamber  16 . Inside the chamber  16  is a workpiece support pedestal  18  including a bias electrode  20 . A plasma bias power generator  22  is coupled through an RF bias impedance match  24  to the bias electrode  20 . 
     An RF enclosure  30  formed of metal is provided above the ceiling  12  and includes a metallic grounded base cylindrical side wall  35  having a top edge  35   a  supporting a shoulder ring  40 , and a conductive top cylindrical side wall  45  extending from the shoulder ring  40  and supporting an overlying conductive cover  50 . The cover  50  and the top cylindrical side wall  45  may foe integrally formed together and may be coupled to RF ground. 
     A process gas supply  51  provides process gas into the chamber  16  through process gas distribution apparatus  52  which may be provided in the side wall  14  (as shown) or in the ceiling  12 , for example. A vacuum pump  53  evacuates the chamber  16  through a pumping port  54 . 
     A floating support plate  55  is located on or slightly above the shoulder ring  40 , and is supported in a manner to be described below. An inductively coupled plasma source power applicator is supported below the support plate  55  by two sets of brackets  60 ,  65  extending downwardly from the support plate  55 . The plasma source power applicator includes a helical inner coil antenna  70  supported and a helical outer coil antenna  75  concentric with the inner coil antenna  70 . The set of brackets  60  support the inner coil antenna  70  while the set of brackets  65  support the outer coil antenna  75 . An RF impedance match box  76  rests on the support plate  55 . A first RF power generator  77  is coupled to the inner coil antenna  70  through impedance match elements (not shown) in the impedance match box  76 . A second RF power generator  78  is coupled to the outer coil antenna  75  through other impedance match elements (not shown) in the impedance match box  76 . 
     A flexible RF gasket  57  provides an RF shield and electrical continuity between the shoulder ring  40  and the floating support plate  55 . The RF gasket  57  may be an annular copper mesh, and may be interrupted to accommodate three support servos described below. 
     The support plate  55  is supported by three support servos  80 ,  85 ,  90  placed at equal (120 degree) intervals on the shoulder ring  40 . The support servos  80 ,  85 ,  90  are identical in one embodiment, and each consists of a support base  100  fastened to a top surface of the shoulder ring  40 , a rail and runner block (rail module)  105  and a servo motor (motor module)  110 . In the illustrated embodiment, the rail and runner block  105  is fastened to the support base  100  while the servo motor  110  is fastened to the rail and runner block  105 , although this relationship may foe modified or reversed in other embodiments. The servo motor  110  rotates a drive pulley  112  and the rail and runner block  105  has a driven pulley  114 , the pulleys  112 ,  114  being linked by a drive belt  116 . The rail and runner block  105  has a vertically actuated, elevator block  118  constrained by a linear vertical internal rail  120  within the rail and runner block  105 , the elevator block  118  being lifted or depressed along the vertical rail  120  depending upon whether the driven pulley  114  is rotated clockwise or counter clockwise. The elevator block includes a radial arm  130  that extends over the floating support plate  55 . A vertical strut  135  supported by the radial arm  130  extends downwardly toward the support plate  55 . A conventional rotatable ball joint  140  is coupled between the strut  135  and the support plate  55 . Movement of the elevator block  118  raises or lowers the portion of the support plate  55  nearest the servo, depending upon whether the elevator block  118  travels up or down. Movement of the support plate  55  in most instances causes the plate  55  to yaw or roll slightly, which in turn causes the ball joint  140  to articulate. Optionally, a limit switch  150  may extend laterally from the elevator block  118 , and upper and lower limit stops  155 ,  160  may actuate the limit switch  150  whenever the elevator block reaches predetermined top and bottom end-of-travel points determined by the locations of the limit stops  155 ,  160 . A control signal cable  170  furnishes electrical control signals and power from a central controller  175  of the reactor of  FIG. 1 . The central controller  175  controls each of the three support servos  80 ,  85 ,  90 . Placement of the three support servos  80 ,  85 ,  90  at equal intervals around the shoulder ring  40  enables the controller  175  to rotate the floating support plate  55  about any tilt axis oriented along any azimuthal angle θ relative to an axis of symmetry of the reactor chamber  16 . 
       FIG. 5  depicts an integration of the reactor of  FIG. 1  into a system for correcting for non-uniformity in etch rate distribution. The system includes a conventional measurement instrument or hardware  400  for measuring etch rate distribution across the surface of a workpiece or semiconductor wafer that has been subjected to a reactive ion etch process in the reactor of  FIG. 1 . A memory  410  stores the etch rate distribution data measured by the hardware  400 . A computer  415  processes the etch rate distribution data stored, in the memory  410  and deduces from that data an azimuthal angle θ defining the major axis of skew in the etch rate data. The computer  415  may further determine, from the magnitude of the skew (difference in etch rates across the major axis of skew) a desired tilt angle α by which the support plate  55  may be rotated about the major axis of skew lying along the angle θ that most likely to correct the skew. A computer  430  computes, from θ and α, the vertical deflection of the elevator block  118  of each of the three servos  80 ,  85 ,  90  that will produce the desired tilt angle α of rotation about the major axis of skew lying along the angle θ. This information is fed to the central controller  175 , which then enables the three servos  80 ,  85 ,  90  to execute the desired motion of the floating support plate  55 . 
       FIG. 6  depicts a method of operating the system of  FIG. 5 . First, a test wafer is processed in the reactor of  FIG. 1  (block  500  of  FIG. 6 ), and an etch rate distribution across the surface of the wafer is obtained (block  510 ). An azimuthal angle θ defining the major axis of skew is inferred from the etch rate distribution (block  520 ). Further, a tilt angle α about the major axis of skew is also inferred from the etch rate distribution (block  530 ) and specifically from the magnitude of the skew, or the difference between etch rates on opposite sides of the major axis of skew. If the magnitude of the skew is below a predetermined threshold or negligible (YES branch of block  535 , then the skew correction process is skipped. Otherwise (NO branch of block  535 , the vertical (Z-axis) movement of each one of the three servos  80 ,  85 ,  90  is computed from α and θ (block  540 ) and the servos are commanded accordingly (block  545 ). A new test wafer replaces the previous test wafer (block  550 ) and the process is repeated. 
     Continuing with the YES branch of block  535 , if the skew magnitude is below a predetermined threshold or is negligible, then skew correction is halted. Any significant etch rate non-uniformity that remains is symmetrical (i.e., radial) so that the controller  175  may now correct the radial non-uniformity by adjusting the apportionment of RF power delivered to the concentric inner and outer coils  70 ,  75  (block  555  of  FIG. 6 ). The controller  175  may perform this correction by directly adjusting the output power levels of the RF power generators  77  and  78 . In one embodiment, for example, the computer  415  may be adapted to deduce the non-uniformity in the radial distribution of etch rate in the data stored in the memory  410 , and further deduce from this information a change in output RF power levels of the two generators  77  and  78 . This change is then conveyed by the computer  415  to the controller  175 , to adjust the output power levels of the RF generators  77  and  78  accordingly. Thereafter, the reactor is prepared to process a production wafer (block  560 ) with minimal or no non-uniformity In etch rate distribution. 
       FIG. 7  depicts an X-Y-Z coordinate system used to locate the three servos  80 ,  85 ,  90 , and defines the angles of rotation θ and α with respect to the X, Y and Z axes. Specifically, the angle θ is a rotation about the Z axis while the angle α is a rotation about the Y axis. The major axis of skew, inferred from the etch distribution data measured on a test wafer, lies in the X-Y plane of  FIG. 7  and is defined with respect to the Y axis by a certain value of the angle θ. Skew correction is performed by tilting the support plate  55  about the skew axis by a particular tilt angle α.  FIG. 8  depicts the X, Y, Z coordinates of the locations of the bail joints of the three servos  80 ,  85 ,  90  in one working example. The vertical motion required for each of the three servos may be computed directly from θ and α. Using the definitions of  FIGS. 7 and 8 , the computer  415  of  FIG. 5  employs the following algorithms to compute the vertical motion of each of the three servos from the angles α and θ in units of inches:
 
 Z (motor 1)=10.2278(−sin α)(cos θ)+5.905(sin α)(sin θ)
 
 Z (motor 2)=10.2278 (sin α)(cos θ)+5.905(sin α)(sin θ)
 
 Z (motor 3)=11.81(−sin α)(sin θ).
 
     The foregoing algorithms were obtained by transforming the vector location of each servo by a rotation about the Z axis through an angle θ and by a rotation about the Y axis by an angle α. 
       FIG. 9  depicts an alterative embodiment, in which the floating support plate  55  of  FIG. 1  is tilted by a single mechanism that replaces the three support servos  80 ,  85 ,  00 . In the embodiment of  FIG. 9 , the radial width of the shoulder ring  40  is enlarged. A floating cradle  600  engages the support plate  55  near the periphery of the support plate  55 . In one embodiment, the support plate  55  may be provided with a radial tab  55   a  at its periphery that engages the floating cradle  600 . A roll axis block  610  is engaged, with the floating cradle  600  and is constrained by a roll axis pin  615  that is fixed to the shoulder ring  40  to rotate about a roll axis  615   a . A roll axis set screw  620  is threadably extends through and is threadably engaged with the roll axis block  610 . The roll axis set screw  620  pushes against a top surface of the shoulder ring  40 , and thereby controls the rotational position of the roll axis block  610  about the roll axis  615   a . A yaw axis block  640  is engaged with the floating cradle  600  and is constrained by a yaw axis pin  650  that is fixed to the shoulder ring  40  to rotate about a yaw axis  650   a . A yaw axis set screw  660  extends through and is threadably engaged with the shoulder ring  40  and pushes against a bottom surface of the yaw axis block  640 , and thereby controls the rotational position of the yaw axis block  640  about the yaw axis  650   a . By rotations of the two screws  620 ,  660 , the support plate  55  may be rotated about a major axis lying along any desired azimuthal angle θ by any desired tilt angle α. The screws  620 ,  660  may be controlled by the controller  175  through actuators  670 ,  680 , respectively. The computer  430  of  FIG. 5  may be programmed to translate desired values of α and θ into corresponding rotations of the screws  620 ,  660 , and the corresponding rotations of the screws  620 ,  660  may be transmitted to the controller  175  to initiate corresponding rotations by the actuators  670 ,  680 . 
     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.