Vertically adjustable chemical mechanical polishing head and method for use thereof

The invention provides a vertically adjustable chemical mechanical polishing head having a pivot mechanism and method for use thereof.

TECHNICAL FIELD

This invention relates generally to chemical mechanical polishing (CMP), and more particularly, but not exclusively, provides a chemical mechanical polishing apparatus having a pivot mechanism and method for use thereof.

BACKGROUND

CMP is a combination of chemical reaction and mechanical buffing. A conventional CMP system includes a polishing head with a retaining ring that holds and rotates a substrate (also referred to interchangeably as a wafer) against a pad surface rotating in the opposite direction or same direction. The pad can be made of cast and sliced polyurethane (or other polymers) with a filler or a urethane coated felt.

During rotation of the substrate against the pad, a slurry of silica (and/or other abrasives) suspended in a mild etchant, such as potassium or ammonium hydroxide, is dispensed onto the pad. The combination of chemical reaction from the slurry and mechanical buffing from the pad removes vertical inconsistencies on the surface of the substrate, thereby forming an extremely flat surface.

However, conventional CMP systems have several shortcomings including process instability that can lead to inconsistent polish profiles of substrates; table-to-table and tool-to-tool variation that can lead to inconsistent polish profiles of substrates processed on different CMP systems; and process optimization difficulties that make it difficult to balance pressure within air-pressurized chambers due to a plurality of pressure controllers.

FIG. lA is a block diagram illustrating a cross section of a prior art polishing head100that exhibits the above-mentioned deficiencies. A retaining ring125is cylindrical in shape and holds a substrate120(also referred to as a wafer) in place during CMP. An air pressure/force balancing method, as indicated by the arrows in FIG. lA, is used to maintain a downward pressing force against a shaft and the substrate120during CMP. In addition, to prevent a plate140from ballooning out of the polishing head100, supplied pressure exerts an upward force.

However, these above-mentioned forces are subject to process instability, which can lead to inconsistent polish profiles of substrates. Specifically, the above-mentioned forces are each powered by air pressure administered by air pressure controllers. The controllers each have their own tolerances that can lead to errors in the amount of air pressure applied. For example, if the pressure in region105is greater than the pressure in region115, the plate140is placed in a position that is lower than expected. A rubber insert130is formed as shown inFIG. 1B(and is different fromFIG. 1Cwhen the plate140is placed in the expected position). In the condition shown inFIG. 1B, the plate140compresses the edge of rubber insert130due to the pressure difference between region105and115. This compressing force gives a pressure on the edge of the substrate120that is different from a pressure on the other region provided by air pressure in region115. As a result, excess pressure is applied on an edge of the substrate120and it increases a polishing rate of the substrate120.

Further, there can be additional variation between conventional CMP systems that lead to inconsistent profiles between substrates. In addition, it can be hard to optimize the process in conventional CMP systems so that the forces required are adequately and consistently balanced.

Another shortcoming of conventional CMP systems is that CMP heads always get lowered to the same position even though the pads wear down over time. This can lead to the insufficient polishing of substrates.

Therefore, a system and method are needed that overcome the above-mentioned deficiencies.

SUMMARY

The invention provides a chemical mechanical polishing head and a method of use thereof. In one embodiment, the chemical mechanical polishing head comprises a substrate holding head and a motor. The motor is coupled to the head and is capable of positioning the head vertically to compensate for pad wear.

In an embodiment of the invention, the method comprises placing a substrate in a chemical mechanical polishing head for polishing and positioning the head to compensate for pad wear.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

FIG. 2is a block diagram illustrating a cross section of polishing head200according to an embodiment of the invention. The polishing head200includes an upper housing215, retaining ring220; retaining ring adapter225; drive flange240; shaft245; ball bearings250; dome255; sub carrier260; rubber insert210; and reference point230.

The retaining ring220is cylindrical in shape and retains a substrate during CMP. The retaining ring220has an inner diameter of at least about 200 mm to about 203 mm for a 200 mm substrate or at least 300 mm to about 303 mm for a 300 mm substrate. The retaining ring220has an outer diameter of about 230 mm to about 275 mm for a 200 mm substrate or about 330 mm to 375 mm for a 300 mm substrate. The retaining ring220is coupled to the upper housing215via a diaphragm (not shown) and the retaining ring adapter225, which has inner and outer diameters substantially similar to the inner and outer diameters of the retaining ring220.

The drive flange240has a bottom surface that is pivotally coupled to the dome255via the ball bearings250. The dome255is coupled to a base flange (not shown). The base flange is also coupled to the sub carrier260and rubber insert210. The reference point230is attached on the sub carrier260and can have a soft pad on the bottom thereof.

The shaft245extends upwards from the drive flange240and is cylindrical in shape. The ball bearings250comprise a plurality of ceramic balls, each having a diameter of about 5/16 of an inch. In an embodiment of the invention, the ball bearings250include fifteen ceramic balls. The dome255is dome shaped with a flat top.

The sub carrier260is cylindrical in shape and has a diameter about equal to the diameter of a substrate (e.g., about 200 mm or about 300 mm). The reference point230is also cylindrical in shape and can have a diameter of just a few millimeters. The rubber insert210forms several air pressure zones or chambers, such as zones280,290, and295, by walling off volume between the rubber insert210and the sub carrier260.

During CMP, the retaining ring220retains a substrate for processing. Pressure is then applied to the drive flange240forcing the polishing head200downwards until a bracket950contacts a stopper assembly945(FIG. 9). Controllable retaining ring air pressure is then supplied to a zone217to force the retaining ring220downwards. Controllable main air pressure is also supplied to zone295. Additional controllable zone air pressure can also be supplied to zones280and290. The main pressure and zone air pressure act to press the rubber insert210against a substrate thereby forcing the substrate to interact with the polishing pad270during CMP. Further, the main pressure and zone pressure place upward pressure on the sub carrier260.

A pivot mechanism (comprising the ball bearings250) enables the pivoting of the polishing head200based on the main pressure and zone pressure. If the shaft245is not assembled vertical to the polishing pad270, the pivot mechanism enables the polishing head200to align parallel to the polishing pad270. The polishing head200can hang a short distance from the drive flange240via 3 springs and 3 pins. Once the polishing head200is placed on the polishing pad270and pressure is applied on the retaining ring220and the back side of the wafer, the upper housing215receives upward force through the base flange (not shown), which is enough to push up the whole polishing head assembly200until the dome255on the top of the polishing head200contacts the ball bearings250coupled to the drive flange240so that the polishing head200can pivot and align in parallel with the polishing pad270. Accordingly, the sub carrier260and the insert210can keep the same vertical position at each polishing.

FIG. 3is a top view of a polishing head300according to an embodiment of the invention. The polishing head300is cylindrical in shape with an outer diameter of about 250 mm for 200 mm substrates or about 350 mm for 300 mm substrates. Different cross-sections of the polishing head300will be discussed in further detail in conjunction withFIG. 4,FIG. 5, FIG.6., andFIG. 7.

The polishing head300comprises a plurality of air pressure inputs, including a center zone input310; an edge zone input305; and a retaining ring input315. The polishing head300also comprises an air channel325and a water channel320. The air pressure inputs305,310and315each independently supply controllable air pressure to different zones within the polishing head300. The retaining ring input315supplies air pressure to a retaining ring zone so as to apply downward pressure on a retaining ring20(FIG. 6) during CMP. The center zone input310supplies air pressure to a center zone within the polishing head300that is formed by an inner rubber insert27(FIG. 6) and a sub carrier38(FIG. 6). The edge zone input305supplies air pressure to the air channel325, which is in communication with an edge zone that is formed by an outer rubber insert28(FIG. 6) and the sub carrier38.

FIG. 4is a cross section illustrating the polishing head300ofFIG. 3. The cross section illustrates a flange drive23; a dome24; ball bearings26; an inner rubber insert27; an outer rubber insert28; a base flange36; and a sub carrier38. The dome24is pivotly coupled to the flange drive23via the ball bearings26. The flange drive23is also cylindrically shaped and pressure applied to the top of the flange drive23forces the polishing head300in a downward direction. The base flange36is cylindrical in shape and is coupled to the bottom of the dome24.

The inner rubber insert27and outer rubber insert28are coupled to the sub carrier38, which in turn is coupled to the base flange36, thereby enabling the inserts27and28to pivotly contact a substrate being acted upon by the polishing head300. The sub carrier38is disk shaped and in conjunction with the inserts27and28form the center zone and edge zone described above. Pressure is supplied to the center zone and edge zone via the center zone input310and edge zone input305, respectively.

FIG. 5is a second cross section illustrating the polishing head300ofFIG. 3. The cross section ofFIG. 5illustrates the coupling of the base flange36to the flange drive23via two assemblies500and510. The first assembly500comprises a collar16; a cap17; a screw2; a rubber cushion22; a washer7and a pin11. The pin11is circumscribed by the collar16and topped with the cap17. In addition, the rubber cushion22is located between the pin11and the collar16so as to cushion the interface between the pin11and the collar16. The washer7is located at the interface between the base flange36and flange drive23and circumscribes the pin11. The first assembly500enables the polishing head300to transfer torque when the shaft rotates the flange drive23.

The second assembly510comprises a washer8; a spring12; a washer9; and a screw33. The screw33couples the base flange36to the flange drive23. The spring12circumscribes the screw33and enables rebound of the base flange36due to pivoting. The second assembly510also includes the washers8and9that are located at the top of the screw33and at the interface between the diaphragm support ring alpha gimbal36and the flange drive23. The second assembly510enables the head300to hang from the flange drive23. It will be appreciated by one of ordinary skill in the art that the polishing head300can include additional assemblies that are substantially similar to the first assembly500and/or second assembly510. For example, in an embodiment of the invention, the polishing head300includes three assemblies substantially similar to the first assembly500and three assemblies substantially similar to the second assembly510.

FIG. 6is a third cross section illustrating the polishing head300ofFIG. 3. Components of the polishing head300that are visible in this cross section include an upper housing37; a seal ring1; a tube30; a screw32; the ceramic balls25; a cross flat countersunk29; the flange drive23; the dome adapter24; the ball holder drive flange25; a retaining ring20; the sub carrier38; the inner rubber insert27; an inner diaphragm support34; the diaphragm support ring alpha gimbal36; the outer rubber insert28; the adapter15; a stop ring21; a lower housing19; a stopper18; and a primary diaphragm35.

The retaining ring20is ring shaped and retains a substrate during CMP. The retaining ring20also circumscribes the disc shaped sub carrier38. Downward pressure is applied to the retaining20to place the retaining ring20in contact with a polishing pad via the retaining ring input315(e.g., tube30).

The retaining ring20is coupled to the diaphragm35with a seal ring1so as to bind the diaphragm35. The outer edge of the diaphragm35is bounded by the upper housing37the lower housing19, the inner edge of the diaphragm35is bounded by the upper housing37and the base flange36, thereby forming a cylindrical chamber capable of receiving pressurized air so that the retaining ring20can exert a downward pressure against the polishing pad.

During CMP, pressure is supplied against the retaining ring20in the retaining ring zone, to the center zone and to the edge zone. The pressures in the center zone and edge zone push the inner rubber insert27and outer rubber insert28downward against the substrate, causing the substrate to interact with the polishing pad. The pressure in the chambers gives the upward force against the dome24via relative parts. Accordingly, the dome24contacts the drive flange23during polishing. Further, the head is enabled to pivot during polishing as a result of the dome and the drive flange23.FIG. 7is a fourth cross section illustrating the polishing head300ofFIG. 3.

FIG. 8is a flowchart illustrating a method800of chemical mechanical polishing. First, a substrate for polishing is loaded (810) into a polishing head, such as polishing head200or300, for polishing. After the substrate has been loaded (810), a slurry is dispensed (820) onto the polishing pad. The slurry can include silica (and/or other abrasives) suspended in a mild etchant, such as potassium or ammonium hydroxide. The polishing head is then placed (830) on the polishing pad.

Air pressure is supplied (840) to the various zones of the polishing head. For example, air can be supplied to zones217and295of the polishing head200. After supplying (840) air pressure, the substrate is rotated (850) against the polishing pad. The combination of chemical reaction from the slurry and mechanical buffing from the pad removes vertical inconsistencies on the surface of the substrate, thereby forming an extremely flat surface.

It will be appreciated that the supplying (840), dispensing (820), and rotating (850) and placing (830) can be performed in an order different from that described above. In addition, it will be appreciated that the dispensing (820), the supplying (840) and the rotating (850) call all be performed substantially simultaneously.

FIGS. 9A-9Dare block diagrams illustrating a polishing system900incorporating a height-adjustable head. The system900includes the head200coupled to a cylindrical shaft930, which travels through a support arm940. A mounting assembly910is fixed to the shaft930and to a sensor assembly920. The support arm940has a stopper assembly945located on a top of the support arm940adjacent and parallel to the shaft930. The stopper assembly945is located on the support arm940in a position that is directly below the sensor assembly920so that the sensor assembly920has a direct unobstructed view of the stopper assembly945.

The sensor assembly920, as shown in more detail inFIG. 9B, includes a sensor960surrounded by a bracket950. The sensor960can include an IR range finder or other sensor (e.g., ultrasound) capable of determining a distance between the sensor960and the top of the stopper assembly945. The sensor960is recessed a distance Z within the bracket950so as to protect the sensor960from damage when the sensor assembly is in contact with the stopper assembly945, as will be discussed in further detail below in conjunction with theFIG. 10. In an embodiment of the invention, Z is equal to about 10 mm.

The stopper assembly945includes a stopper coupled to a servomotor (not shown) that is located within the support arm940. The servomotor moves the stopper in a vertical direction from a low position, as shown inFIG. 9Aup to a height of Y−Z+X above the low position. The servomotor can also move the head200in a vertical direction. Y is the distance between the sensor960and the stopper when the head200is positioned to compress the insert210against the sub carrier260as shown inFIG. 9C. The value of Y decreases slightly after each substrate120polishing due to pad wear. For example, Y can decrease by about 0.3 μm to up to about 10.0 μm per substrate120polishing. Depending on the sensitivity of the servomotor, Y can be measured after every CMP process or after a certain number of intervals. For example, if the servomotor is capable of raising the stopper to a position with an accuracy of 50 μm, then Y can be calculated after every 10 to 50 CMP processes.

X is the distance between the sub carrier260and the insert210during polishing as shown inFIG. 9D, i.e., the height of the zone295. In an embodiment of the invention, X is equal to about 0.5 mm.

It will be appreciated by one of ordinary skill in the art that the system900can use different polishing heads, such as heads100or300.

FIG. 10is a block diagram illustrating the polishing system900in an uncompressed state, i.e., in position for CMP. After the sensor960measures Y, the head200is raised so that the bottom of the sensor assembly920is positioned at a height above the stopper assembly945equal to Y−Z+X. The servomotor then raises the stopper so that the top of the stopper is located at Y−Z+X above the original lowered stopper position. The head200is then lowered, if necessary, to a CMP position until the sensor assembly920contacts the stopper. It will be appreciated that a CMP position can be obtained by adjusting the vertical position by a servo motor without using a stopper. Also, vertical distance will be measured by a pulse signal from the servo motor instead of using the sensor.

In an embodiment of the invention, the head200can be lowered to different heights during different steps of the CMP. For example, where total polishing time is set to 100 seconds and comprises three different polishing sequences at different heights, the first could be set for 30 seconds with polishing condition A, the second could move to polishing condition B for 60 seconds and the last to polishing condition C for 10 seconds. In a Cu circuit process, Cu metal is first removed on the circuit and then a barrier metal below the Cu is removed. The materials on both the Cu and the barrier layer are different and therefore use a different slurry and conditions for removing each material. Therefore, 2 or more different conditions (polishing step) are set in the Cu process. The vertical position of the polishing head is a parameter that determines polishing performance and needs to change between the Cu and barrier layer polishing steps. As a result, vertical position is not fixed in one position during whole polishing but fixed during each polishing step.

FIG. 11is a block diagram illustrating an example computer1100capable of controlling the polishing system900. The example computer1100can be located within the support arm940or at any other location and is communicatively coupled, via wired or wireless techniques, to the servomotor and to the sensor960. Use of the computer1100to control the servomotor and the sensor960will be discussed further below in conjunction withFIG. 12. The example computer1100includes a central processing unit (CPU)1105; working memory1110; persistent memory1120; input/output (I/O) interface1130; display1140and input device1150, all communicatively coupled to each other via a bus1160. The CPU1105may include an INTEL PENTIUM microprocessor, a Motorola POWERPC microprocessor, or any other processor capable to execute software stored in the persistent memory1120. The working memory1110may include random access memory (RAM) or any other type of read/write memory devices or combination of memory devices. The persistent memory1120may include a hard drive, read only memory (ROM) or any other type of memory device or combination of memory devices that can retain data after the example computer1100is shut off. The I/O interface1130is communicatively coupled, via wired or wireless techniques, to the sensor960and the servomotor. The display1140, like other components of the computer1100, is optional and may include a cathode ray tube display or other display device. The input device1150, which is also optional, may include a keyboard, mouse, or other device for inputting data, or a combination of devices for inputting data.

One skilled in the art will recognize that the example computer1100may also include additional devices, such as network connections, additional memory, additional processors, LANs, input/output lines for transferring information across a hardware channel, the Internet or an intranet, etc. One skilled in the art will also recognize that the programs and data may be received by and stored in the system in alternative ways. Further, in an embodiment of the invention, an ASIC is used in placed of the computer1100to control the servomotor and the sensor960.

FIG. 12is a block diagram illustrating a positioning system1200, which can be resident on the example computer1100. The positioning system1200communicates with the sensor960and the servomotor and controls movement of the sensor960and the head200via control of the servomotor. The positioning system1200includes a sensor engine1210, a servomotor engine1220, a head engine1230, and a parameters file1240. The sensor engine1210controls the sensor960including turning the sensor960on and off to get a distance reading. The servomotor engine1220controls the vertical movement of the stopper and the head200in response to calculations made by the head engine1230. The head engine1230calculates the position the head200should be in for CMP based on readings from the sensor960and values stored in the parameters file1240. The parameters file1240stores values X and Z. In an embodiment of the invention X and Z are equal to about 0.5 mm and 10 mm, respectively.

In an embodiment of the invention, the parameters file1240can also include a maximum Y value that corresponds with the maximum pad wear. The head engine1230can compare the measured Y value with the maximum Y value to determine if Y exceeds the maximum Y value. If the measured Y does exceed the maximum Y, the head engine1230can alert an operator of the system900that the pad270has exceeded the maximum pad wear and the operator can then replace the pad270with a new pad before initiating CMP.

In another embodiment of the invention, the parameters file1240includes pad wear rate data, which is calculated by measuring the difference in pad height between consecutive polishings. Alternatively, the pad wear data rate can be calculated by measuring the difference in pad height between a first polishing and a later polishing (e.g., 50th) and dividing the difference by the number of polishings between measurements. The parameters file1240, in this embodiment, can also hold a head height for polishing when using a new polishing pad. Accordingly, depending on the sensitivity of the servomotor, the head engine1230can then use the pad wear rate data to recalculate the proposed position of the head200for every polishing after a pre-specified number of polishings. For example, the head position could be calculated as the original head height (when using a new polishing pad) less the pad wear rate times the number of polishings.

In another embodiment of the invention, the parameters file1240also stores vertical positioning information for different steps during a polishing process. For example, as described above, the head could be positioned at a first height for polishing Cu and then positioned at a second height for polishing a barrier layer.

FIG. 13is a flowchart illustrating a method1300of positioning a CMP head200. First, a substrate120is placed (1310) in the head200. Next, the head200is lowered (1320) so as to compress the sub carrier260against the insert210. The distance is then measured (1330) between the sensor960and the top of the stopper assembly945to yield the value Y. It is then determined (1340) if the value Y exceeds a maximum Y value. If it does, then the operator is warned (1350) via aural, visual, tactile and/or other techniques that pad wear exceeds recommended amounts and the method1300ends. Otherwise, the head200is then raised (1360) and the stopper is raised (1370) to a height above its lowered position equal to Y−Z+X. The head200is then lowered (1380) until the sensor assembly920contacts the stopper assembly945. CMP can then begin (1390). In an embodiment of the invention, CMP (1390) can comprise different steps that adjust the vertical position of the head200to polish different layers of the substrate120. The method1300then ends.

FIG. 14is a flowchart illustrating a second method1400of positioning a CMP head200. First, the system is initialized (1410), which can include calculating a pad wear rate and determining the compressibility of the head (i.e., the distance X). The pad wear rate can be calculated by measuring the difference in pad height between consecutive polishings. Alternatively, the pad wear rate can be calculated by measuring the difference in pad height between a first polishing and a later polishing (e.g., 50th) and dividing the difference by the number of polishings between measurements. The compressibility of the pad can be measured by measuring the height of the head before and after compressing it against a polishing pad.

After initialization (1410), a substrate is placed (1420) in the head for polishing. The stopper is then positioned (1430), e.g., raised, so that when the head is lowered (1440) it is positioned to compensate for pad wear. The positioning can be calculated by subtracting the pad wear rate times the number of polishings from the original head height. After positioning (1430) the stopper, the head is lowered (1440) until the sensor assembly contacts the stopper. CMP then begins (1450) and the method1400ends.

The foregoing description of the illustrated embodiments of the present invention is by way of example only, and other variations and modifications of the above-described embodiments and methods are possible in light of the foregoing teaching. For example, the embodiments described herein are not intended to be exhaustive or limiting. The present invention is limited only by the following claims.