Patent Application
20120009847
1. Field of the Invention
Embodiments of the present invention generally relate to methods of chemical mechanical polishing a substrate.
2. Description of the Related Art
Chemical mechanical polishing (CMP) is a common technique used to planarize substrates. CMP utilizes two modes to planarize substrates. One mode is a chemical reaction using a chemical composition, typically a slurry or other fluid medium, for removal of material from substrates, and the other is mechanical force. In conventional CMP techniques, a substrate carrier or polishing head is mounted on a carrier assembly and positioned in contact with a polishing pad in a CMP apparatus. The carrier assembly provides a controllable pressure to the substrate urging the substrate against the polishing pad. The pad is moved relative to the substrate by an external driving force. Thus, the CMP apparatus affects a polishing or rubbing movement between the substrate surface and the polishing pad, while dispensing a polishing composition to encompass both chemical and mechanical activities. However, as more substrates are polished in the CMP apparatus the efficiency of the polishing pad changes, and the polishing pad eventually requires replacement. Polishing pads and polishing slurries can be costly components, and it is preferable to reduce the amount of downtime required to replace polishing pads. Therefore, it is preferable to decrease the frequency at which pads are replaced, and to utilize polishing slurries more efficiently.
Thus, there is a need for a method of polishing a substrate using a polishing pad with longer useful life, and efficiently utilizing polishing slurry delivered thereto.
Embodiments of the present invention generally relate to methods for chemical mechanical polishing a substrate. The methods generally include measuring the thickness of a polishing pad having grooves or other slurry transport features on a polishing surface. Once the depth of the grooves on the polishing surface is determined, a flow rate of a polishing slurry is adjusted in response to the determined groove depth. A predetermined number of substrates are polished on the polishing surface. The method can then optionally be repeated.
In one embodiment, a method includes measuring a thickness of a polishing pad having grooves disposed in a polishing surface of the polishing pad. A depth of the grooves disposed in the polishing surface is determined, and a flow rate of a polishing slurry introduced to the polishing surface is adjusted in response to the determined depth of the grooves disposed in the polishing surface. A predetermined number of substrates is polished, and the method is repeated.
In another embodiment, a method includes measuring a thickness of a polishing pad having grooves disposed in a polishing surface of the polishing pad. The measured thickness of the polishing pad is compared to an initial pre-polish thickness of the polishing pad to determine a reduction in the polishing pad thickness. A depth of the grooves disposed in the polishing surface is then calculated. A flow rate of a polishing slurry introduced to the polishing surface is adjusted in response to the calculated depth of the grooves disposed in the polishing surface. A predetermined number of substrates are then polished. The polishing comprises contacting each of the predetermined number of substrates to the polishing pad, and introducing the polishing slurry to the polishing pad at the adjusted flow rate for each of the predetermined number of substrates. The method is then repeated.
In another embodiment, a method includes measuring a thickness of a polishing pad having grooves disposed in a polishing surface. The measured thickness of the polishing pad is compared to an initial pre-polish thickness of the polishing pad to determine a reduction in polishing pad thickness, and a depth of the grooves disposed in the polishing surface is calculated. The calculated depth of the grooves is compared to a value stored in a look-up table. A slurry is introduced to the polishing pad at a predetermined flow rate, and the predetermined flow rate is dependent upon the calculated depth of the grooves. A predetermined number of substrates are polished, and then the method is repeated.
So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical 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.
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 disclosed in one embodiment may be beneficially utilized on other embodiments without specific recitation.
Embodiments of the present invention generally relate to methods for chemical mechanical polishing a substrate. The methods generally include measuring the thickness of a polishing pad having grooves or other slurry transport features on a polishing surface. Once the depth of the grooves on the polishing surface is determined, a flow rate of a polishing slurry is adjusted in response to the determined groove depth. A predetermined number of substrates are polished on the polishing surface. The method can then optionally be repeated.
While the particular apparatus in which the embodiments described herein can be practiced is not limited, it is particularly beneficial to practice the embodiments in a REFLEXION GT™ system, REFLEXION® LK CMP system, and MIRRA MESA® system sold by Applied Materials, Inc., of Santa Clara, Calif. Additionally, CMP systems available from other manufacturers may also benefit from embodiments described herein.
The polishing module 106 includes a plurality of polishing stations 124 on which substrates are polished while retained in one or more carrier heads 126a, 126b. The polishing stations 124 are sized to interface with two or more carrier heads 126a, 126b simultaneously so that polishing of two or more substrates may occur using a single polishing station 124 at the same time. The carrier heads 126a, 126b are coupled to a carriage (not shown) that is mounted to an overhead track 128 that is shown in phantom in
Two polishing stations 124 are shown located in opposite corners of the polishing module 106. At least one load cup 122 is in the corner of the polishing module 106 between the polishing stations 124 closest the wet robot 108. The load cup 122 facilitates transfer between the wet robot 108 and the carrier heads 126a, 126b. Optionally, a third polishing station 124 (shown in phantom) may be positioned in the corner of the polishing station 124 opposite the load cups 122. Alternatively, a second pair of load cups 122 (also shown in phantom) may be located in the corner of the polishing module 106 opposite the load cups 122 that are positioned proximate the wet robot. Additional polishing stations 124 may be integrated in the polishing module 106 in systems having a larger footprint.
Each polishing station 124 includes a polishing pad having a polishing surface 130 capable of polishing at least two substrates at the same time. The polishing pad may be formed from polyurethane. Each of the polishing stations 124 also includes a pad conditioning assembly 140. In one embodiment, the pad conditioning assembly 140 may comprise a conditioning head 132 which dresses the polishing surface 130 of the polishing pad by removing polishing debris and opening the pores of the pad, and a polishing fluid delivery arm 134. In one embodiment, each polishing station 124 comprises multiple pad conditioning assemblies 140. The polishing pad is supported on a platen assembly which rotates the polishing surface 130 during processing. The polishing surface 130 is suitable for at least one of a chemical mechanical polishing and/or an electrochemical mechanical polishing process. The system 100 is coupled with a power source 180.
To facilitate control of the polishing system 100 and processes performed thereon, a controller 190 comprising a central processing unit (CPU) 192, memory 194, and support circuits 196, is connected to the polishing system 100. The CPU 192 may be one of any form of computer processor that can be used in an industrial setting for controlling various drives and pressures. The memory 194 is connected to the CPU 192. The memory 194, or computer-readable medium, may be one or more of readily available memory such as random access memory (RAM), read only memory (ROM), floppy disk, hard disk, or any other form of digital storage, local or remote. The support circuits 196 are connected to the CPU 192 for supporting the processor in a conventional manner. These circuits include cache, power supplies, clock circuits, input/output circuitry, subsystems, and the like.
CMP polishing systems, such as polishing system 100 shown in
As the polishing pad having a groove depth of 40 mils polishes substrates, the groove depth is reduced and the polishing rate increases. It would be wasteful to continue supplying a constant amount of polishing slurry since the polishing rate is already increasing due to pad wear. Thus, embodiments of the present invention provide methods for closed-loop control of polishing slurry flow rate in response to a measured groove depth of a polishing pad.
The support assembly 246 is adapted to position the conditioning head 232 in contact with the polishing surface 230, and further is adapted to provide a relative motion therebetween. The support arm 244 has a distal end coupled to the conditioning head 232 and a proximal end coupled to the base 247. The base 247 rotates to sweep the conditioning head 232 across the polishing surface 230 to condition the polishing surface 230. The polishing surface 230 of the polishing pad 236 has grooves 201 disposed therein. As a result of the relative motion of the conditioning head 232 with respect to the polishing surface 230 of the polishing pad 236, the displacement sensor 260 may take thickness measurements of the polishing surface 230 and the polishing pad 236.
The sensor coupled to the conditioning arm allows a thickness of the polishing pad 236 to be measured at various points during a portion of a normal operation cycle, while the accompanying logic allows the measurement data to be captured and displayed. In some embodiments, the displacement sensor 260 may utilize an inductive sensor.
In embodiments where the displacement sensor 260 is a laser based sensor, the thickness of the polishing pad 236 is measured directly. The support arm 244 is in a fixed position with respect to the platen assembly 241, and the laser is in a fixed position with respect to the arm. Consequently, the laser is in a fixed position with respect to the platen assembly 241. By measuring the distance to the processing pad and calculating the difference between the distance to the polishing pad 236 and the distance to the platen assembly 241, the remaining thickness of the polishing pad 236 may be determined. In some embodiments, the resolution of the thickness measurement using the laser-based displacement sensor 260 may be within 25 um.
In embodiments where the displacement sensor 260 is an inductive sensor, the thickness of the polishing pad 236 is measured indirectly. The support arm 244 is actuated around a pivot point until the conditioning head 232 comes in contact with the processing pad 236. An inductive sensor, which emits an electromagnetic field, is mounted to the end of the pivot based conditioning support arm 244. In accordance with Faraday's law of induction, the voltage in a closed loop is directly proportional to the change in the magnetic field per change in time. The stronger the applied magnetic field, the greater the eddy currents developed and the greater the opposing field. A signal from the sensor is directly related to the distance from the tip of the sensor to the metallic platen assembly 241. As the platen assembly 241 rotates the conditioning head 232 rides on the surface of the pad and the inductive sensor rises and falls with the conditioning support arm 244 according to the profile of the polishing pad 236. As the inductive sensor gets closer to the metallic platen assembly 241 the voltage of the signal increases, which is an indication of processing pad wear. The signal from the sensor is processed and captures the variation in the thickness of the polishing pad 236. In some embodiments, the resolution of the thickness measurement using the inductive sensor 260 may be within 1 um.
By comparing the measured polishing pad thickness to the initial pre-polish pad thickness, a change in pad thickness can be determined by the controller. Since the polishing pad wears away at the polishing surface, the difference in pad thickness also directly relates to a change in the depth of the grooves disposed in the polishing surface. For example, the polishing pad thickness and the groove depth are reduced at the same rate. Thus, by measuring a change in polishing pad thickness, a change in groove depth can also be determined. Additionally, since the number and location (or pitch) of grooves is fixed during pad fabrication, and since typical polishing pads have grooves with approximately vertical sidewalls, the determined groove depth can also be used to calculate groove volume.
In step 384, the closed-loop controller adjusts the polishing slurry flow rate provided to the polishing pad in response to the polishing pad groove depth determined in step 382. Typically, prior to beginning a polishing process, a user-input look-up table is stored in the controller. The look-up table correlates a predetermined flow rate of polishing slurry to a determined or measured groove depth. The flow rates may vary from process to process, and may depend upon pad composition, slurry composition, or substrate material. Typically, the flow rates are determined empirically.
In step 386, the polishing slurry is provided to the polishing pad at the corresponding flow rate for each substrate processed thereafter, and a predetermined number of substrates are subsequently polished. For example, 200, 300, 500 or 1000 wafers may be consecutively polished using the flow rate of polishing slurry corresponding to the last determined polishing pad groove depth. In optional step 388, steps 380, 382, 384, and 386 are repeated. Although steps 380, 382, 384, and 386 may be repeated more often than every few hundred substrates, for example, after every substrate is polished, it is generally not advantageous to do so. Typically, there is not enough of a change in groove depth to cause a noticeable change in polishing slurry flow rate after polishing only a single substrate. Therefore, it is sufficient to determine polishing pad groove depth and adjust polishing slurry flow rate after at least about 500 substrates have been polished. However, as the polishing pad thickness decreases and the polishing pad approaches its process end-life, it may be desirable to measure polishing pad thickness and determine groove depth more frequently. This is due to the fact that polishing pads experience a “spike” in polishing removal rate near the end of the useful life of the pad. By measuring the pad thickness more often, it can be easier to identify when the pad needs replacement. Additionally, more frequent pad measurements can help to avoid substrate damage caused by over-polishing during the “spike,” and also help to avoid wasting excess consumables such as polishing slurry.
In comparison, the polishing pad having groove depths of 40 mils only removes about 1400 angstroms per minute of material when the polishing slurry flow rate is 200 milliliters per minute. When the slurry flow rate is increased to 300 milliliters per minute, the polishing pad having groove depths of 40 mils is capable of removing about 1600 angstroms per minute. The polishing pad having groove depths of 40 mils requires a slurry flow rate of 400 milliliters per minute to produce a removal rate of about 1750 angstroms per minute, which is approximately what a pad with a groove depth of 30 mils removes with a slurry flow rate of 200 milliliters per minute.
Additionally, methods herein can be used for pads having an initial groove depth of less than 40 mils, for example, about 30 mils. However, groove depth is only a secondary factor affecting polishing rate when the groove depth is less than 30 mils. A stronger relationship between groove depth and removal rate can be seen when the groove depth is greater than 30 mils; therefore, it is of greater concern to utilize closed-loop control of slurry flow when using pads having groove depths greater than 30 mils.
A closed-loop control system allows a constant polishing rate to be maintained while reducing the amount of slurry delivered to the polishing pad as the pad wears. Closed-loop control of slurry flow allows for the use of pads having a longer useful life, which reduces the frequency at which pads need to be replaced. Additionally, closed-loop control of slurry flow in response to pad thickness allows the slurry flow rate to be tailored to the pad thickness and groove depth, which helps ensure the excess slurry is not being wastefully provided to the pad. The closed-loop control of polishing slurry should lead to significant cost savings over the life of the polishing pad by not wasting consumable materials. Embodiments disclosed herein allow for efficient use of thicker polishing pads with longer useful lives, while still maintaining a high substrate throughput.
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.
