US20030220049A1

US20030220049A1 - Conditioning disk actuating system

Info

Publication number: US20030220049A1
Application number: US10/152,470
Authority: US
Inventors: Chih-Ming Chang
Original assignee: Taiwan Semiconductor Manufacturing Co TSMC Ltd
Current assignee: Taiwan Semiconductor Manufacturing Co TSMC Ltd
Priority date: 2002-05-21
Filing date: 2002-05-21
Publication date: 2003-11-27

Abstract

A conditioning disk actuating system for raising and lowering a conditioning disk inside a conditioning head for the conditioning of semiconductor wafer polishing pads. The system includes a fluid-actuated cylinder which is coupled to a travel hub vertically slidably mounted in a travel housing provided inside the conditioning head. The conditioning disk is mounted on the bottom end of a disk shaft carried by the travel hub. The fluid-actuated cylinder is operated to selectively lower and raise the travel hub and conditioning disk to press the disk against the polishing pad and remove the disk from the polishing pad, respectively. A position sensing mechanism may be provided in the conditioning head for revealing the “up” or “down” position of the conditioning disk.

Description

FIELD OF THE INVENTION

The present invention relates to disks used in the conditioning of polishing pads on chemical mechanical polishers for semiconductor wafers. More particularly, the present invention relates to a new and improved actuating system for raising and lowering a polishing pad conditioning disk in a pad conditioning head of a chemical mechanical polisher.

BACKGROUND OF THE INVENTION

Apparatus for polishing thin, flat semiconductor wafers are well-known in the art. Such apparatus normally includes a polishing head which carries a membrane for engaging and forcing a semiconductor wafer against a wetted polishing surface, such as a polishing pad. Either the pad or the polishing head is rotated and oscillates the wafer over the polishing surface. The polishing head is forced downwardly onto the polishing surface by a pressurized air system or similar arrangement. The downward force pressing the polishing head against the polishing surface can be adjusted as desired. The polishing head is typically mounted on an elongated pivoting carrier arm, which can move the pressure head between several operative positions. In one operative position, the carrier arm positions a wafer mounted on the pressure head in contact with the polishing pad. In order to remove the wafer from contact with the polishing surface, the carrier arm is first pivoted upwardly to lift the pressure head and wafer from the polishing surface. The carrier arm is then pivoted laterally to move the pressure head and wafer carried by the pressure head to an auxiliary wafer processing station. The auxiliary processing station may include, for example, a station for cleaning the wafer and/or polishing head, a wafer unload station, or a wafer load station.

More recently, chemical-mechanical polishing (CMP) apparatus has been employed in combination with a pneumatically actuated polishing head. CMP apparatus is used primarily for polishing the front face or device side of a semiconductor wafer during the fabrication of semiconductor devices on the wafer. A wafer is “planarized” or smoothed one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer is polished by being placed on a carrier and pressed face down onto a polishing pad covered with a slurry of colloidal silica or alumina in deionized water.

A schematic of a typical CMP apparatus is shown in FIGS. 1A and 1B. The

apparatus

20 for chemical mechanical polishing consists of a rotating wafer holder 14 that holds the wafer 10, the appropriate slurry 24, and a polishing pad 12 which is normally mounted to a rotating table 26 by adhesive means. The polishing pad 12 is applied to the wafer surface 22 at a specific pressure. The chemical mechanical polishing method can be used to provide a planar surface on dielectric layers, on deep and shallow trenches that are filled with polysilicon or oxide, and on various metal films.

CMP polishing results from a combination of chemical and mechanical effects. A possible mechanism for the CMP process involves the formation of a chemically altered layer at the surface of the material being polished. The layer is mechanically removed from the underlying bulk material. An altered layer is then regrown on the surface while the process is repeated again. For instance, in metal polishing, a metal oxide may be formed and removed separately.

A polishing pad is typically constructed in two layers overlying a platen with the resilient layer as the outer layer of the pad. The layers are typically made of polyurethane and may include a filler for controlling the dimensional stability of the layers. The polishing pad is usually several times the diameter of a wafer and the wafer is kept off-center on the pad to prevent polishing a non-planar surface onto the wafer. The wafer is also rotated to prevent polishing a taper into the wafer. Although the axis of rotation of the wafer and the axis of rotation of the pad are not collinear, the axes must be parallel.

In a CMP head, large variations in the removal rate, or polishing rate, across the whole wafer area are frequently observed. A thickness variation across the wafer is therefore produced as a major cause for wafer non-uniformity. In the improved CMP head design, even though a pneumatic system for forcing the wafer surface onto a polishing pad is used, the system cannot selectively apply different pressures at different locations on the surface of the wafer. This effect is shown in FIG. 1C, i.e. in a profilometer trace obtained on an 8-inch wafer. The thickness difference between the highest point and the lowest point on the wafer is almost 2,000 angstroms, resulting in a standard deviation of 472 angstroms, or 6.26%. The curve shown in FIG. 1C is plotted with the removal rates in the vertical axis and the distance from the center of the wafer in the horizontal axis. It is seen that the removal rates obtained at the edge portions of the wafer are substantially higher than the removal rates at or near the center of the wafer. The thickness uniformity on the resulting wafer after the CMP process is poor.

The

polishing pad

12 is a consumable item used in a semiconductor wafer fabrication process. Under normal wafer fabrication conditions, the polishing pad is replaced after about 12 hours of usage. Polishing pads may be hard, incompressible pads or soft pads. For oxide polishing, hard and stiffer pads are generally used to achieve planarity. Softer pads are generally used in other polishing processes to achieve improved uniformity and smooth surfaces. The hard pads and the soft pads may also be combined in an arrangement of stacked pads for customized applications.

A problem frequently encountered in the use of polishing pads in oxide planarization is the rapid deterioration in oxide polishing rates with successive wafers. The cause for the deterioration is known as “pad glazing”, wherein the surface of a polishing pad becomes smooth such that slurry is no longer held in between the fibers of the pad. This physical phenomenon on the pad surface is not caused by any chemical reactions between the pad and the slurry.

To remedy the pad glazing effect, numerous techniques of pad conditioning or scrubbing have been proposed to regenerate and restore the pad surface and thereby restore the polishing rates of the pad. The pad conditioning techniques include the use of silicon carbide particles, diamond emery paper, blade or knife for scraping or scoring the polishing pad surface. The goal of the conditioning process is to remove polishing debris from the pad surface and re-open pores in the pad by forming micro-scratches in the surface of the pad for improved pad lifetime. The pad conditioning process can be carried out either during a polishing process, i.e. known as concurrent conditioning, or after a polishing process.

Referring next to FIG. 2, a

conventional CMP apparatus

50 includes a conditioning head 52 fitted with a conditioning disk 68, which is formed by embedding or encapsulating diamond particles in nickel coated on the surface of the conditioning disk 68; a polishing pad 56; and a slurry delivery arm 54 positioned over the polishing pad 56. The conditioning head 52 is mounted on a conditioning arm 58 which is extended over the top of the polishing pad 56 for making a sweeping motion across the entire surface of the polishing pad 56. The slurry delivery arm 54 is equipped with slurry dispensing nozzles 62 which are used for dispensing a slurry solution on the top surface 60 of the polishing pad 56. Surface grooves 64 are further provided in the top surface 60 to facilitate even distribution of the slurry solution and to help entrapping undesirable particles that are generated by coagulated slurry solution or any other foreign particles which have fallen on top of the polishing pad 56 during a polishing process. The surface grooves 64, while serving an important function of distributing the slurry, also presents a processing problem when the pad surface 60 gradually wears out after prolonged use.

As illustrated in FIGS. 3 and 4, the

conventional conditioning head

52 typically includes an air cavity 72 in which is slidably disposed a typically rubber diaphragm 70. The conditioning disk 68 is attached to a disk shaft 80 extending through the diaphragm 70. An air intake tube 74 and an air vacuum tube 76 are provided in fluid communication with the air cavity 72, and each is connected to an air/vacuum source 78 such as an SCM Venturi air/vacuum pump. Accordingly, to facilitate conditioning the polishing pad 56, pressurized air is introduced from the air/vacuum source 78 through the air intake tube 74 and into the air cavity 72, where the air presses downwardly against the diaphragm 70, which in turn presses the conditioning disk 68 against the polishing pad 56 to score and condition the polishing pad 56 as the disk shaft 80 rotates the conditioning disk 68. The conditioning operation is terminated by withdrawing air from the air cavity 72 through the air vacuum tube 76, wherein the resulting reduced air pressure in the air cavity 72 raises the diaphragm 70 and withdraws the conditioning disk 68 from contact with the polishing pad 56.

One of the problems encountered in operation of the

conventional conditioning head

52 is frequent rupturing or reduction in elasticity of the diaphragm 70 after prolonged use. Consequently, pressurized air in the air cavity 72 tends to escape the conditioning head 52 through the diaphragm 70 upon an attempt to press the conditioning disk 68 against the polishing pad 56 for conditioning of the polishing pad 56. Further, the vacuum pressure in the air cavity 72 is dispelled by leakage of air into the conditioning head 52 through the ruptured diaphragm 70 upon an attempt to withdraw the conditioning disk 68 from the polishing pad 56. The reduction in air pressure in the air cavity 72 results in inadequate downward force of the conditioning disk 68 against the polishing pad 56 to effectively condition the polishing pad 56. As a result, the diaphragm 70 and other components of the conditioning head 52 or vacuum system must be frequently replaced, and the replacement operation may require 3-4 hours to complete, resulting in substantial down time. Further, since the conditioning head 52 lacks any sensing mechanism to reveal the position of the conditioning disk 68 therein, personnel operating the conditioning head 52 are incapable of readily determining whether the conditioning disk 68 is in the “up” position of FIG. 3 or the “down” position of FIG. 4.

Accordingly, a more durable system is needed for raising and lowering a conditioning disk in a conditioning head of a chemical mechanical polisher to prevent the need for frequent replacement of parts.

An object of the present invention is to provide a new and improved conditioning disk actuating system for lowering and raising a conditioning disk in a conditioning head.

Another object of the present invention is to provide a conditioning disk actuating system for lowering and raising a conditioning disk in a conditioning head, which system includes durable parts to provent the need for frequent replacement of parts.

Still another object of the present invention is to provide a system for reliably lowering and raising a conditioning disk in a conditioning head over a period of prolonged operation.

Yet another object of the present invention is to provide a system which is capable of sensing the “up” or “down” position of a conditioning disk inside a conditioning head for conditioning wafer polishing pads.

Another object of the present invention is to provide a conditioning disk actuating system which is capable of sustaining a stable downward force against a conditioning disk during conditioning of a polishing pad using the conditioning disk.

SUMMARY OF THE INVENTION

In accordance with these and other objects and advantages, the present invention comprises a new and improved conditioning disk actuating system for raising and lowering a conditioning disk inside a conditioning head for the conditioning of semiconductor wafer polishing pads. The system includes a fluid-actuated cylinder which is coupled to a travel hub vertically slidably mounted in a travel housing provided inside the conditioning head. The conditioning disk is mounted on the bottom end of a disk shaft carried by the travel hub. The fluid-actuated cylinder is operated to selectively lower and raise the travel hub and conditioning disk to press the disk against the polishing pad and remove the disk from the polishing pad, respectively. A position sensing mechanism may be provided in the conditioning head for revealing the “up” or “down” position of the conditioning disk.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example, with reference to the accompanying drawings, in which: [0021]
FIG. 1A is a cross-sectional view of a conventional chemical mechanical polishing apparatus; [0022]
FIG. 1B is an enlarged, cross-sectional view of a section of a wafer and polishing pad with a slurry solution therein between, in a conventional disk polishing operation; [0023]
FIG. 1C is a graph illustrating the changes in removal rates as a function of distance on a wafer after a polishing pad is repeatedly used; [0024]
FIG. 2 is a perspective view of a conventional CMP polishing pad with a slurry dispensing arm and a conditioning disk positioned on top; [0025]
FIG. 3 is a schematic view illustrating interior components of a conventional conditioning head, with the diaphragm and conditioning disk components of the conditioning head shown in the “up” position; [0026]
FIG. 4 is a schematic view illustrating interior components of a conventional conditioning head, with the diaphragm and conditioning disk components of the conditioning head shown in the “down” position; [0027]
FIG. 5A is a schematic view illustrating a conditioning disk actuating system of the present invention, with the conditioning disk component of the conditioning head thereof shown in the “up” position; [0028]
FIG. 5B is a schematic view illustrating a conditioning disk actuating system of the present invention, with the conditioning disk component of the conditioning head thereof shown in the “down” position; [0029]
FIG. 6 is a cross-sectional view of a disk shaft component typically used in attaching a conditioning disk to the conditioning head of the present invention, more particularly illustrating an illustrative, threaded technique for mounting the conditioning disk; [0030]
FIG. 7 is an enlarged sectional view of the travel hub component of the present invention, more particularly illustrating an illustrative sensor mechanism for sensing the “up” and “down” positions of the conditioning disk in the conditioning head; and [0031]
FIG. 8 is a schematic illustrating an illustrative control system for the present invention.[0032]

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is directed to a conditioning disk actuating system for selectively lowering and pressing a conditioning disk of a conditioning head against a polishing pad for conditioning of the polishing pad and raising the conditioning disk from contact with the polishing pad after the conditioning operation. The system of the present invention is generally indicated by [0033] reference numeral 28 and includes a conditioning head 29 mounted on the end of an elongated conditioning arm 85. The conditioning head 29 includes a head interior 30 through which a disk shaft 33 extends. The upper end of the disk shaft 33 may be conventionally fitted with a belt gear (not illustrated) which engages a belt and motor (not illustrated) for rotation of the disk shaft 33 inside the head interior 30. A travel housing 31 is mounted in the bottom of the interior 30 and includes an inwardly-extending housing flange 39. A flange interior 43 is defined by the housing flange 39. A travel hub 32 is mounted on the disk shaft 33 and is stationary with respect to the disk shaft 33 as the disk shaft 33 rotates therein. Accordingly, ball bearings (not illustrated) or other mechanism may be provided at the interface of the travel hub 32 and the disk shaft 33 to facilitate smooth rotation of the disk shaft 33 in the travel hub 32. The travel hub 32 is mounted for vertical movement with the disk shaft 33 inside the flange interior 43. A bottom plate 35 is mounted on the bottom surface of the travel housing 31 and is fitted with a central plate opening 49 (FIG. 5B). When the travel hub 32 and disk shaft 33 are disposed in the uppermost position inside the flange interior 43, the plate opening 49 accommodates a conditioning disk 36 which is typically removably mounted on the bottom end of the disk shaft 33, as illustrated in FIG. 5A. When the travel hub 32 and disk shaft 33 are disposed in the lowermost position inside the flange interior 43, the conditioning disk 36 is displaced downwardly from the plate opening 49, as illustrated in FIG. 5B. As illustrated in FIG. 6, the conditioning disk 36 is typically removably attached to the bottom end of the disk shaft 33 by threading nipple threads 38 on an attachment nipple 37 extending upwardly from the conditioning disk 36 with companion shaft threads 34 inside the disk shaft 33.
A [0034] magnetic position sensor 88, which may be conventional, may be mounted against the travel housing 31 in the flange interior 43. A magnetic ring 90 circumscribes the travel hub 32 in adjacent contact with the position sensor 88. As illustrated in FIG. 7, the position sensor 88 is connected to an alarm or other positioning monitor 92, which may be conventional, typically by means of wiring 93. Accordingly, magnetic attraction between the position sensor 88 and the magnetic ring 90 along the various locations on the position sensor 88 is interpreted by the positioning monitor 92 with regard to the vertical location of the travel hub 32 and thus, the disk shaft 33 in the flange housing 43. This, in turn, readily indicates to operating personnel whether the conditioning disk 36 is in the upper position of FIG. 5A or the lower, operational position of FIG. 5B. It is understood that any suitable alternative vertical positioning monitor known by those skilled in the art may be used to monitor the position of the travel hub 32 inside the flange interior 43 and thus, the “up” or “down” position of the conditioning disk 36.
As illustrated in FIGS. 5A and 5B, the [0035] system 28 of the present invention further includes a fluid-actuated cylinder 44 typically mounted on the bottom portion 86 of the conditioning arm 85. A piston 48 slidably disposed in the housing 45 of the fluid-actuated cylinder 44 is attached to the lower end of a piston shaft 82. A top fluid hose 46 and a bottom fluid hose 47 extend from the housing 45 and are connected to a fluid source 95, which may be a source of compressed clean, dry air (CDA), or alternatively, a pump and supply mechanism for hydraulic fluid. The piston shaft 82 extends upwardly from the piston 48 inside the housing 45 and through an opening (not illustrated) in the bottom portion 86 of the conditioning arm 85. A link coupling 83 is provided on the upper end of the piston shaft 82 inside the conditioning arm 85.
As further illustrated in FIGS. 5A and 5B, a [0036] link 40 connects the link coupling 83 to the travel hub 32. The link 40 typically includes a horizontal segment 41 which extends horizontally from the link coupling 83 and a vertical segment 42 which extends downwardly from the extending end of the horizontal segment 41, through a link opening (not illustrated) provided in the housing flange 39 of the travel hub 32, and is attached to the travel hub 32 typically by means of a standard base bearing. Accordingly, by operation of the fluid source 95 to introduce fluid under pressure into the fluid-actuated cylinder 44 through the top fluid hose 46, the fluid pushes downwardly against the top surface of the piston 48, sliding the piston 48 downwardly in the housing 45. Consequently, the piston shaft 82 lowers the link coupling 83, which lowers the link 40 through the link opening in the housing flange 39. This action lowers the travel hub 32 and disk shaft 33 inside the flange interior 43, and the disk shaft 33 in turn lowers the conditioning disk 36 from the plate opening 49 and against the upper surface of a polishing pad 97 for conditioning thereof, as illustrated in FIG. 5B and hereinafter further described. Conversely, by operation of the fluid source 95 to introduce fluid under pressure into the fluid-actuated cylinder 44 through the bottom fluid hose 47, the fluid pushes upwardly against the bottom surface of the piston 48, thereby sliding the piston 48 upwardly in the housing 45. Consequently, the piston shaft 82 raises the link coupling 83, which raises the link 40 through the link opening in the housing flange 39. This action raises the travel hub 32 and disk shaft 33 inside the flange interior 43, and the disk shaft 33 in turn raises the conditioning disk 36 from the upper surface of the polishing pad 97 and again positions the conditioning disk 36 in the plate opening 49 of the bottom plate 35, as illustrated in FIG. 5A.
Referring again to FIGS. 5A and 5B, the [0037] system 28 of the present invention is operated to condition a polishing pad 97 supported on a platen 98 of a chemical mechanical polisher (not shown). The conditioning head 29 of the present invention is initially positioned over the surface of the polishing pad 97, and as the platen 98 rotates the polishing pad 97 thereon, a polishing slurry (not illustrated) is deposited on the surface of the rotating polishing pad 97. Simultaneously, as the disk shaft 33 and attached conditioning disk 36 are rotated in the conditioning head 29, the fluid-actuated cylinder 44 is operated via the fluid source 95 to lower the travel hub 32 and disk shaft 33 in the conditioning head 29 from the position illustrated in FIG. 5A to the position illustrated in FIG. 5B, in the manner heretofore described. Accordingly, as the conditioning head 29 is moved horizontally across the surface of the polishing pad 97 by pivoting action of the conditioning arm 85, in conventional fashion, the rotating conditioning disk 36 scores and shears the surface of the polishing pad 97 to condition the polishing pad 97 in conventional fashion. It will be appreciated by those skilled in the art that the fluid source 95 and fluid-actuated cylinder 44 are capable of applying the conditioning disk 36 against the polishing pad 97 at a steady pressure throughout the conditioning operation.
To terminate the conditioning operation, the [0038] rotating conditioning disk 36 is removed from contact with the polishing pad 97 by operating the fluid-actuated cylinder 44 to raise the piston 48 in the housing 45 and thus, raise the travel hub 32 and disk shaft 33 in the conditioning head 29 and remove the conditioning disk 36 from contact with the polishing pad 97. It will be appreciated by those skilled in the art that the positioning monitor 92, in conjunction with the position sensor 88 on the stationary travel housing 31 and the magnetic ring 90 on the travel hub 32, provide a reliable indicator to operating personnel of the relative position of the conditioning disk 36 with respect to the polishing pad 97 throughout the conditioning operation.
Referring next to FIG. 8, a schematic is shown illustrating a typical pneumatic control valve system in triplicate for each of three conditioning disk actuating systems of the present invention, located at three respective chemical mechanical polishers (not illustrated) in a semiconductor fabrication facility. In the embodiment of the present invention wherein the [0039] fluid source 95 is a source of clean, dry air (CDA), the CDA source 95 is pneumatically connected to each of three “up” control valves 4 typically through each of three needle valves 2 that can be used to control the upward speed of each travel hub in the corresponding travel housing. Each “up” control valve 4 facilitates flow of CDA into the fluid-actuated cylinder 44 through the bottom fluid hose 47 (FIGS. 5A and 5B) thereof to facilitate raising the travel hub 32 and disk shaft 33 in each conditioning head 29. A “down” control valve 6 facilitates flow of CDA into the fluid-actuated cylinder 44 through the top fluid hose 46 thereof to facilitate lowering the travel hub 32 and disk shaft 33 in each conditioning head 29 and pressing the conditioning disk 36 against the polishing pad 97. Typically, the “down” control valves 6 are components of the conventional diaphragm-actuated control system (such as the SCM Venturi air/vacuum system) having the conditioning head 52 heretofore described with respect to FIGS. 2-4. Accordingly, the fluid-actuated cylinders 44 and the “up” control valves 4 of the present invention may be retrofitted to the “down” control valves 6 as part of the conventional diaphragm-actuated control system. It is understood that the schematic of FIG. 8 represents only one example of a pneumatic valve control system which is suitable for controlling the conditioning disk actuating system of the present invention.
While the preferred embodiments of the invention have been described above, it will be recognized and understood that various modifications can be made in the invention and the appended claims are intended to cover all such modifications which may fall within the spirit and scope of the invention. [0040]
Having described my invention with the particularity set forth above, I claim: [0041]

Claims

What is claimed is:

1. A system comprising:

a conditioning head;

a travel hub vertically movably mounted in said conditioning head;

a disk shaft carried by said travel hub;

a conditioning disk carried by said disk shaft;

a fluid-actuated cylinder operably engaging said travel hub for selectively moving said travel hub between upper and lower positions in said conditioning head; and

a fluid source connected to said fluid-actuated cylinder for flowing a fluid to and from said fluid-actuated cylinder.

2. The system of claim 1 further comprising a position sensing mechanism operably engaging said travel hub for sensing a position of said travel hub in said conditioning head.

3. The system of claim 1 wherein said fluid-actuated cylinder comprises a pneumatic cylinder.

4. The system of claim 3 further comprising a position sensing mechanism operably engaging said travel hub for sensing a position of said travel hub in said conditioning head.

5. The system of claim 1 further comprising a travel housing provided in said conditioning head and wherein said travel hub is vertically movably mounted in said travel housing.

6. The system of claim 5 further comprising a position sensing mechanism operably engaging said travel hub for sensing a position of said travel hub in said conditioning head.

7. The system of claim 5 wherein said fluid-actuated cylinder comprises a pneumatic cylinder.

8. The system of claim 7 further comprising a position sensing mechanism operably engaging said travel hub for sensing a position of said travel hub in said conditioning head.

9. The system of claim 2 wherein said position sensing mechanism comprises a magnetic ring carried by said travel hub, a position sensor provided in said conditioning head in adjacent contact with said magnetic ring, and a positioning monitor operably connected to said position sensor.

10. The system of claim 9 wherein said fluid-actuated cylinder comprises a pneumatic cylinder.

11. The system of claim 9 further comprising a travel housing provided in said conditioning head and wherein said travel hub is vertically movably mounted in said travel housing and said position sensor is provided on said travel housing.

12. The system of claim 3 further comprising a needle valve interposed between said fluid source and said fluid actuated cylinder for controlling a speed of movement of said fluid actuated cylinder from said lower position to said upper position.

13. A system comprising:

a conditioning head;

a travel hub vertically movably mounted in said conditioning head;

a disk shaft carried by said travel hub;

a conditioning disk carried by said disk shaft;

a link carried by said travel hub;

a fluid-actuated cylinder engaging said link for selectively moving said travel hub between upper and lower positions in said conditioning head; and

14. The system of claim 12 further comprising a position sensing mechanism operably engaging said travel hub for sensing a position of said travel hub in said conditioning head.

15. The system of claim 12 further comprising a travel housing provided in said conditioning head and wherein said travel hub is vertically movably mounted in said travel housing.

16. The system of claim 14 wherein said position sensing mechanism comprises a magnetic ring carried by said travel hub, a position sensor provided in said conditioning head in adjacent contact with said magnetic ring, and a positioning monitor operably connected to said position sensor.

17. A system comprising:

a conditioning head;

a travel hub vertically movably mounted in said conditioning head;

a disk shaft carried by said travel hub;

a conditioning disk carried by said disk shaft;

a link having a vertical segment carried by said travel hub and a horizontal segment extending from said vertical segment;

a fluid-actuated cylinder engaging said horizontal segment of said link for selectively moving said travel hub between upper and lower positions in said conditioning head; and

18. The system of claim 17 wherein said fluid-actuated cylinder comprises a pneumatic cylinder.

19. The system of claim 17 further comprising a position sensing mechanism operably engaging said travel hub for sensing a position of said travel hub in said conditioning head.

20. The system of claim 19 wherein said position sensing mechanism comprises a magnetic ring carried by said travel hub, a position sensor provided in said conditioning head in adjacent contact with said magnetic ring, and a positioning monitor operably connected to said position sensor.