Attorney Docket No 44021362WO01; 05542-1556WO1 CLEANING OF CMP TEMPERATURE CONTROL SYSTEM TECHNICAL FIELD The present disclosure relates to chemical mechanical polishing (CMP), and more specifically to a temperature control apparatus for CMP and cleaning of a temperature control apparatus. BACKGROUND An integrated circuit is typically formed on a substrate by the sequential deposition of conductive, semiconductive, or insulative layers on a semiconductor wafer. A variety of fabrication processes require planarization of a layer on the substrate. For example, one fabrication step involves depositing a filler layer over a non-planar surface and planarizing the filler layer. For certain applications, the filler layer is planarized until the top surface of a patterned layer is exposed. For example, a metal layer can be deposited on a patterned insulative layer to fill the trenches and holes in the insulative layer. After planarization, the remaining portions of the metal in the trenches and holes of the patterned layer form vias, plugs, and lines to provide conductive paths between thin film circuits on the substrate. As another example, a dielectric layer can be deposited over a patterned conductive layer, and then planarized to enable subsequent photolithographic steps. Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier head. The exposed surface of the substrate is typically placed against a rotating polishing pad. The carrier head provides a controllable load on the substrate to push it against the polishing pad. A polishing slurry with abrasive particles is typically supplied to the surface of the polishing pad. In addition, the temperature of the polishing pad can be controlled, e.g., by directing a heated or cooled fluid onto the surface of the polishing pad. SUMMARY In one aspect, a chemical mechanical polishing apparatus has a platen to hold a polishing pad, a carrier to hold a substrate against a polishing surface of the polishing pad during a polishing process, a heating system, a purge gas source, a purge liquid source, and a controller. The heating system includes a source of heated gas, an arm extending over the platen, and a manifold with an a plurality of openings positioned over the platen and separated from the polishing pad for delivering the heated gas onto the polishing pad. Attorney Docket No 44021362WO01; 05542-1556WO1 The controller is configured to cause the heated gas to flow from the source of heated gas through the manifold and the plurality of openings to heat the polishing pad during a polishing operation, and to cause the apparatus to perform a purging operation which alternates between flowing purge gas from the purge gas source and flowing purge liquid from the purge liquid source through the manifold and the plurality of openings. In another aspect, a chemical mechanical polishing apparatus has a platen to hold a polishing pad, a carrier to hold a substrate against a polishing surface of the polishing pad during a polishing process, a heating system, and a controller. The heating system includes an arm extending over the platen and having a manifold with a plurality of openings positioned over the platen and separated from the polishing pad for delivering the heated gas onto the polishing pad, a source of heating liquid, and a boiler having an inlet for the heating liquid and a steam outlet coupled by a fluid delivery line to the manifold in the arm. The controller is configured to cause the heated gas to flow from the boiler through the manifold and the plurality of openings to heat the polishing pad during a polishing operation, and to cause the apparatus to perform a purging operation which alternates between filling and draining the boiler with the heating liquid. In another aspect, a chemical mechanical polishing apparatus has a platen to hold a polishing pad, a carrier to hold a substrate against a polishing surface of the polishing pad during a polishing process, a source of heated gas, and an arm extending over the platen. The arm including a diffusion plate that separates a volume inside the arm into an upper plenum that is coupled to the source of heated gas and a lower plenum, and a distribution plate that provides a lower surface of the arm and that has a plurality of openings disposed uniformly across the distribution plate, and wherein the diffusion plate includes a plurality of apertures distributed non-uniformly across the diffusion plate so as to induce a non-uniform mass flow of the heated gas through the plurality of openings. Implementations may include, but are not limited to, one or more of the following possible advantages. The cleaning procedure can reduce build-up of contaminants, e.g., scale build up, in the polishing pad heating system. This can reduce the likelihood of such contaminants reaching the substrate and causing defects, e.g., scratching. The cleaning procedure can be conducted during idle time of the polishing machine, and thus avoid significantly impacting throughput. The use of an internal diffusion plate in the heating manifold permits establishment of different flow rates of the heating fluid to different regions of the polishing pad, which can help establish a desired temperature Attorney Docket No 44021362WO01; 05542-1556WO1 profile on the polishing pad, e.g., a more uniform temperature profile, which can improve polishing uniformity. The internal diffusion plate permits the heating fluid dispensing plate to have small apertures, which can reduce the likelihood of splashback entering the heating manifold. Temperature variation over a polishing operation can be reduced. This can improve predictability of polishing the polishing process. Temperature variation from one polishing operation to another polishing operation can be reduced. This can improve wafer-to-wafer uniformity and improve repeatability of the polishing process. Plates with different patterns of apertures can be swapped into fluid dispenser to provide different temperature profiles. This permits quick testing for different temperature profiles or modification of a polisher for a process that requires a new temperature profile. The details of one or more implementations are set forth in the accompanying drawings and the description below. Other aspects, features, and advantages will be apparent from the description and drawings, and from the claims. BRIEF DESCRIPTION OF THE DRAWINGS FIG.1 illustrates a schematic cross-sectional view of an example of a polishing apparatus. FIG.2 illustrates a schematic top view of an example chemical mechanical polishing apparatus. FIG.3 illustrates flow chart of a process for cleaning a heating delivery arm. Like reference symbols in the various drawings indicate like elements. DETAILED DESCRIPTION Chemical mechanical polishing operates by a combination of mechanical abrasion and chemical etching at the interface between the substrate, polishing liquid, and polishing pad. Both the chemical-related variables in a CMP process, e.g., as the initiation and rates of the participating reactions, and the mechanical-related variables, e.g., the surface friction coefficient and viscoelasticity of the polishing pad, are strongly temperature dependent. Consequently, variation in the surface temperature of the polishing pad can result in changes in removal rate, polishing uniformity, erosion, dishing, and residue. By more tightly controlling the temperature of the surface of the polishing pad during polishing, variation in temperature can be reduced, and polishing Attorney Docket No 44021362WO01; 05542-1556WO1 performance, e.g., as measured by within-wafer non-uniformity or wafer-to-wafer non- uniformity, can be improved. Some techniques have been proposed for temperature control. As one example, a temperature-controlled medium, e.g., a liquid, vapor or spray, can be dispensed onto the polishing surface of the polishing pad (or the polishing liquid on the polishing pad). In particular, a dispensing arm can extend over the polishing pad, and the temperature- controlled medium, e.g., steam, can be dispensed from the bottom of the arm onto the polishing surface to heat the polishing pad. Moreover, it can be desirable to provide non- uniform mass flow of the temperature control medium along the radius of the polishing pad. For example, annular regions of the pad further from the axis of rotation have larger surface area, and thus will require greater mass flow. However, a problem that can occur is contamination of the manifold that delivers the temperature control fluid to the polishing pad, e.g., build-up of minerals on interior or exterior surfaces of the manifold. For example, scale build up is a result of hardened calcium and mineral deposits that dry and affix to surfaces. One possible source of contamination is splash-back of polishing chemistry from the polishing pad. For example, impact of the temperature control medium, e.g., steam, on the polishing liquid can cause some droplets of the polishing liquid to be flung back toward the dispensing arm. If such droplets pass through a dispensing nozzle on the bottom of the arm, chemistry can accumulate in the distribution manifold in the interior of the arm. A technique to at least partially address this problem is to have the bottom of the arm be provided by a heating diffusion plate with small holes through which the temperature control medium, e.g., steam, will pass to contact the polishing pad. The small holes reduce the likelihood of splash-back liquid from entering the interior of the distribution manifold. The holes can be disposed with uniform density along the length of the dispensing arm. In order to establish the non-uniform mass flow, the manifold can include an interior diffusion plate having apertures arranged with non-uniform spacing along the length of the dispensing arm. As a result, pressure and mass flow through the small holes in the dispensing plate will be higher below regions of the diffusion plate with a higher density of apertures. Another possible source of contamination is contaminants from the temperature control medium itself. Ultra-pure water, e.g., semiconductor grade water having 1-2 Attorney Docket No 44021362WO01; 05542-1556WO1 contaminating molecules per million molecules of water, can be used in a boiler to generate steam. However, even with ultra-pure water, over time the contaminating molecules can accumulate on the interior surfaces of boiler or the distribution manifold, and thus form scale. Two techniques to at least partially address this problem are 1) to perform a purging operation in which gas and cleaning fluid are alternately directed through the distribution manifold to flush out contaminants in the distribution manifold, and 2) periodically drain all water from the boiler and refill with fresh ultrapure water. These two techniques can be used independently or in conjunction, and can also be used independently or in conjunction with the dispensing arm having an interior diffusion plate. FIGS.1 and 2 illustrate an example of a polishing station of a chemical mechanical polishing system 20. The polishing system 20 includes a rotatable disk- shaped platen 24 on which a polishing pad 30 is situated. The platen 24 is operable to rotate (see arrow A in FIG.2) about an axis 25. For example, a motor 22 can turn a drive shaft 28 to rotate the platen 24. The polishing pad 30 can be a two-layer polishing pad with an outer polishing layer 34 and a softer backing layer 32. The polishing station 20 can include a supply port 39 to dispense a polishing liquid 38, such as an abrasive slurry, onto the polishing pad 30. The exact location of the supply port 39 may vary between different implementations, but typically, the supply port 39 is positioned at the end of an arm near the center of the polishing pad 30. For example, the supply port 39 can be positioned at the end of a slurry supply arm 170 (see FIG.2). Alternatively, the supply port 39 can be positioned at the end of a heating delivery arm 110. The polishing station 20 can include a pad conditioner apparatus 90 with a conditioning disk 92 (see FIG.2) to maintain the surface roughness of the polishing pad 30. The conditioning disk 90 can be positioned at the end of an arm 94 that can swing so as to sweep the disk 90 radially across the polishing pad 30. A carrier head 70 is operable to hold a substrate 10 against the polishing pad 30. The carrier head 70 is suspended from a support structure 72, e.g., a carousel or a track, and is connected by a drive shaft 74 to a carrier head rotation motor 76 so that the carrier head can rotate about an axis 71. Optionally, the carrier head 70 can oscillate laterally, Attorney Docket No 44021362WO01; 05542-1556WO1 e.g., on sliders on the carousel, by movement along the track, or by rotational oscillation of the carousel itself. The carrier head 70 can include a flexible membrane 80 having a substrate mounting surface to contact the back side of the substrate 10, and a plurality of pressurizable chambers 82 to apply different pressures to different zones, e.g., different radial zones, on the substrate 10. The carrier head can also include a retaining ring 84 to hold the substrate. In some implementations, the polishing station 20 includes a temperature sensor 64 to monitor a temperature in the polishing station or a component of/in the polishing station, e.g., the temperature of the polishing pad and/or slurry on the polishing pad. For example, the temperature sensor 64 could be an infrared (IR) sensor, e.g., an IR camera, positioned above the polishing pad 30 and configured to measure the temperature of the polishing pad 30 and/or slurry 38 on the polishing pad. In particular, the temperature sensor 64 can be configured to measure the temperature at multiple points along the radius of the polishing pad 30 in order to generate a radial temperature profile. For example, the IR camera can have a field of view that spans the radius of the polishing pad 30. In some implementations, the temperature sensor is a contact sensor rather than a non-contact sensor. For example, the temperature sensor 64 can be thermocouple or IR thermometer positioned on or in the platen 24. In addition, the temperature sensor 64 can be in direct contact with the polishing pad. In some implementations, multiple temperature sensors could be spaced at different radial positions across the polishing pad 30 in order to provide the temperature at multiple points along the radius of the polishing pad 30. This technique could be used in the alternative or in addition to an IR camera. Although illustrated in FIG.1 as positioned to monitor the temperature of the polishing pad 30 and/or slurry 38 on the pad 30, the temperature sensor 64 could be positioned inside the carrier head 70 to measure the temperature of the substrate 10. The temperature sensor 64 can be in direct contact (i.e., a contacting sensor) with the semiconductor wafer of the substrate 10. In some implementations, multiple temperature sensors are included in the polishing station 22, e.g., to measure temperatures of different components of/in the polishing station. Attorney Docket No 44021362WO01; 05542-1556WO1 The polishing system 20 also includes a temperature control system 100 (see FIG. 2) to control the temperature of the polishing pad 30 and/or slurry 38 on the polishing pad. The temperature control system 100 can include a heating system 102 and/or a cooling system 104 (see FIG.2). At least one, and in some implementations both, of the cooling system 104 and heating system 102 operate by delivering a temperature- controlled fluid 130, e.g., a liquid, vapor or spray, onto the polishing surface 36 of the polishing pad 30 (which can be either directly onto the polishing surface, or onto a thin layer of liquid that is already present on the polishing surface). For the heating system 102, a dispensing arm 120 extends over the platen 24 and polishing pad 30. An interior of the dispensing arm 120 includes a heating manifold to distribute a heating fluid 140 from an inlet 122 to a plurality of apertures 124 in the bottom surface of the dispensing arm 120 and onto the polishing surface 36. The heating fluid 140 can be a gas, e.g., steam or heated air. The gas can, but need not, carry an aerosolized liquid. The heating fluid 140 is above room temperature, e.g., at 40-120º C, e.g., at 90-110º C, when the heating fluid reaches the polishing surface 36. The heating fluid 140 can be water, such as substantially pure de-ionized water, or water that includes additives or chemicals. In some implementations, for the heating operation, the heating system 102 directs dry steam onto the polishing pad, e.g., the heating fluid 140 consists of dry steam without additives or chemicals or aerosolized liquid. Where the heating fluid 140 is steam, the steam can be generated at a boiler 110. Steam from the boiler 110 can flow through a fluid delivery line 118, which can be provided by piping, flexible tubing, passages through solid bodies, or some combination thereof, to the dispensing arm 120. The boiler can include a thermally insulated canister 160 having a lower portion surrounded by heating elements 162, e.g., resistive heating coils. A steam outlet 164 that leads to the valve 110a is positioned in the upper portion of the canister 160. An inlet/outlet 166 at the bottom of the canister 160 permits liquid, e.g., ultrapure water, to be pumped into the canister 160 from a DI water source 170, e.g., if a valve 170a is opened, or for liquid in the canister 160 to be drained through a drain outlet 172, e.g., if a valve 172a is opened. The heating system 102 can also include a purge gas supply 112 to provide a purge gas, e.g., N ₂ , and optionally a purge liquid supply 114 to provide a purge liquid, Attorney Docket No 44021362WO01; 05542-1556WO1 e.g., deionized (DI) water. The DI water source 114 can also provide the DI water source 170. water and N ₂ gas are typically available from supply pipes in a semiconductor fabrication facility. Purge gas and purge liquid can also flow through the fluid delivery line 118 to the dispensing arm 120. Each of the boiler 110, purge gas supply 112, and a purge liquid supply 114 can be coupled by an associated valve 110a, 112a, 114a to the fluid delivery line 118, and a controller 90 can control opening and closing of the valves to control which fluid flows to the dispensing arm 120, as discussed further below. The dispensing arm 120 extends over the platen 24 and polishing pad 30 from an edge of the polishing pad to or at least near (e.g., within 5% of the total radius of the polishing pad) the center of polishing pad 30. The arm 120 can be supported by a base 121, and the base 121 can be supported on the same frame 40 as the platen 24. The base 121 can include one or more actuators, e.g., a linear actuator to raise or lower the arm 120, and/or a rotational actuator to swing the arm 120 laterally over the platen 24. The arm 120 is positioned to avoid colliding with other hardware components such as the polishing head 70 and the pad conditioning disk 92. The dispensing arm 120 can be generally linear and can have a substantially uniform width along its length, although other shapes such as a circular sector (aka a “pie slice”), an arc or triangular wedge (all as bottom views of the arm) can be used to achieve a desired effectiveness in temperature control of the polishing pad 30. For example, the dispensing arm 120 can be curved, e.g., form an arc or a portion of a spiral. Multiple openings 124 are formed through a dispensing plate 126 that provides the bottom surface of the dispensing arm 120. Each opening 124 is oriented to direct the heating fluid 140 onto the polishing pad 30, e.g., for a cylindrical hole the central axis can be normal to the polishing surface. The openings 124 can be relatively small, e.g., 0.25 to 1 mm in diameter. Such relatively small openings can reduce the likelihood of splash- back liquid to enter the interior of the distribution manifold. The openings 124 are uniformly sized and are distributed with a uniform density in the dispensing plate 126. In some implementations, there are three rows of openings 124 that extend lengthwise along the dispensing arm 120, with the rows staggered. As the dispensing plate 126 is prefabricated, the size and position of the openings 124 is set, e.g., not adjustable during a polishing operation. Attorney Docket No 44021362WO01; 05542-1556WO1 The arm 120 can be supported by the base 121 so that the openings 124 are separated from the polishing pad 30 by a gap 128. The gap 128 can be 0.5 to 5 mm. In particular, the gap can be selected such that the heat of the heating fluid does not significantly dissipate before the fluid reaches the polishing pad. For example, the gap 128 can be selected such that steam emitted from the openings does not condense before reaching the polishing pad. To provide the non-uniform mass flow, a diffusion plate 130 is positioned inside the arm 120 to divide the manifold into an upper plenum 132 and a lower plenum 134. The fluid delivery line 118 is coupled and delivers fluid, e.g., the heating fluid 140, to the upper plenum 132. The dispenser arm 120 can have a single inlet 122 through which the various fluids, e.g., heating medium, purge gas and liquid, enters the upper plenum 132. The inlet 122 can be located at a distal end of the arm 120 relative to the axis of rotation of the platen 24. A plurality of apertures 136 are formed through the diffusion plate 130 to fluidically couple the upper plenum 132 to the lower plenum 134. The apertures 136 can be uniformly sized. The apertures 136 can be wider than the openings 124, although this is not required. For example, the apertures 136 can be 0.5 to 3 cm across. Unlike the openings 124, the apertures 136 are disposed with non-uniform density along the length of the polishing arm 120. In particular, the apertures 136 are disposed at greater density with increasing distance from the rotational axis 25 of the platen 24. Thus, the apertures 136 are more closely spaced at the end of arm that is near the center of the polishing pad 30 than at the end of the arm that joins the base 121. This establishes a pressure gradient within the lower plenum 134 such that pressure in the lower plenum 134 increases with distance from the rotational axis 25. As a result, the mass flow of the heating fluid 140 through the openings 124 increases with distance from the rotational axis 25. The pattern of apertures 136 in the diffusion plate 130 can be designed to meet the specific needs of various temperature control profiles. In some cases, the temperature control profile can define mass flow rates of the heated fluid flow onto the polishing pad as a function of radial distance from an axis of rotation of the platen. For example, the mass flow rate can increase parabolically with distance from the axis of rotation. Attorney Docket No 44021362WO01; 05542-1556WO1 Rather than having uniform size and non-uniform distribution, the apertures 136 can be uniformly distributed but have non-uniform size to accomplish a similar mass flow profile for the heating fluid 140. As the distribution plate 126 and diffusion plate 130 are prefabricated with set positions and sizes for the openings 124 and apertures 136, the size of the openings 124 and apertures 136 does not change during a polishing operation. However, to change the distribution of heating fluid 140, the distribution arm 120 can be removed and a new diffusion plate 130 with a different pattern of apertures swapped in. Thus, different plates with different patterns of openings can be used to provide different temperature profiles for different polishing operations. This also permits quick testing for different temperature profiles or modification of a polisher for a process that requires a new temperature profile. For example, a radial temperature profile during polishing of a substrate without temperature control by the arm can be measured. The a pattern of openings that will provide a mass flow profile to compensate for non-uniformity in the radial temperature profile is calculated, e.g., as an inverse of the radial temperature profile. A base plate having openings arranged in the pattern can be fabricated or selected from a set of pre- fabricated base-plates. Then the base plate is installed in the arm and used during polishing of a substrate. FIGS.1 and 2 are schematic and not necessarily illustrative of the number of openings or apertures. In particular, there could be a larger number of openings, e.g., fifty to two-hundred openings. Moreover, although FIGS.2 illustrates circular openings, the openings could be rectangular, e.g., square, hexagonal, or other shapes. Optionally, some of the openings 124 can be oriented so that a central axis of the spray from that opening is at an oblique angle relative to the polishing surface 36. The heated fluid, e.g., steam, can be directed from one or more of the openings 124 to have a horizontal component in a direction opposite to the direction of motion of polishing pad 30 in the region of impingement as caused by rotation of the platen 24. For the cooling system 104, the coolant can be a gas, e.g., air, or a liquid, e.g., water. The coolant can be at room temperature or chilled below room temperature, e.g., at 0-15º C. In some implementations, the cooling system 104 uses a spray of air and liquid, e.g., an aerosolized spray of liquid, e.g., water. In particular, the cooling system can have nozzles that generate an aerosolized spray of water that is chilled below room Attorney Docket No 44021362WO01; 05542-1556WO1 temperature. In some implementations, the spray can include a solid material, e.g., ice particles formed by passing water droplets through a de Laval nozzle or the like. The coolant can be delivered by flowing through one or more apertures 152, e.g., holes or slots, optionally formed in nozzles, in a coolant delivery arm 150. The apertures can be provided by a manifold that is connected to a coolant source 154. The arm 150 can be constructed similarly to the arm 120 of the heating system, Along the direction of rotation of the platen 24, the arm 150 of the cooling system 104 can be positioned between the arm 120 of the heating system 102 and the carrier head 70. Along the direction of rotation of the platen 24, the arm 150 of the cooling system 104 can be positioned between the arm 120 of the heating system 102 and the slurry delivery arm 170. For example, the arm 150 of the cooling system 104, the arm 120 of the heating system 102, the slurry delivery arm 170, and the carrier head 70 can be positioned in that order along the direction rotation of the platen 24. Although FIG.2 illustrates separate arms for each subsystem, e.g., the heating system 102, cooling system 104 and rinse system 106, various subsystems can be included in a single assembly supported by a common arm. For example, an assembly can include a cooling module, a rinse module, a heating module, a slurry delivery module, and optionally a wiper module. Each module can include a body, e.g., an arcuate body, that can be secured to a common mounting plate, and the common mounting plate can be secured at the end of an arm so that the assembly is positioned over the polishing pad 30. Various fluid delivery components, e.g., plenums, tubing, passages, etc., can extend inside each body. In some implementations, the modules are separately detachable from the mounting plate. Each module can have similar components to carry out the functions of the arm of the associated system described above. Returning to FIG.1, the polishing system 20 also include the controller 90 to control operation of various components, e.g., the temperature control system 100. The controller 90 can be coupled to the boiler 110 to control the temperature and rate of generation of the steam. Similarly, the controller 90 can be coupled to a valve 156 (see FIG.2) to control flow rates of coolant from the coolant source 154, to the coolant source 154 to control the temperature of the coolant, and to the valves 110a, 112a, 114a, to control flow of the heating fluid 150, the purge gas, and the purge liquid. One or more valves can be provided by one or more liquid flow controllers (LFCs). Attorney Docket No 44021362WO01; 05542-1556WO1 The controller 90 can be configured to receive the temperature measurements from the temperature sensor 64. The controller 90 can compare the measured temperature to a desired temperature, and generate a feedback signal to a control mechanism (e.g., actuator, power source, pump, valve, etc.) for the flow rate of the respective heating and coolant fluids. The feedback signal is used by the controller 90, e.g., based on an internal feedback algorithm, to cause the control mechanism to adjust the amount of cooling or heating such that the polishing pad and/or slurry reaches (or at least moves closer to) the desired temperature. As discussed above, contaminants can build up on the inside of the manifold, e.g., upper plenum 132 and lower plenum 134, in the fluid distribution arm 120 of the heating system 102. FIG.3 is a flow chart to illustrate a process 300 of performing a purging operation to 300 to flush out contaminants in the distribution manifold. In brief, the polishing system 20 alternates between directing the purge gas through the manifold (306) and directing the purge liquid through the manifold (304). As the manifold begins filled with steam or atmosphere, initially, the purge liquid is flowed until all the gas initially in the delivery line 118 and manifold is purged out through the openings 124 (302). Then the polishing system alternates between flowing the purge gas (304) and flowing the purge liquid (306). These two steps can be repeated five to twenty times. The purge gas may be flowed for 5-20 seconds, and the purge liquid can be flowed for 5- 20 seconds. In some implementations, there is a 50% duty cycle, i.e., the steps of flowing the purge gas and purge liquid have equal duration. A potential advantage of pulsing between gas and liquid at short durations of 5-20 seconds, as opposed to simply flowing the purge liquid or gas continuously, is improved cleanliness. Without being limited to theory, the gas pulses can force the purge liquid at high velocity, and the leading edge of the purge liquid can be more effective at removing contaminants from the walls of the manifold. Once steps 304 and 306 have been repeated the requisite number of times, a final purge gas step can be performed (308). In this step, the purge gas is flowed until all of the purge liquid that was in the delivery line 118 and manifold is purged out through the openings 124. The purge gas can be purified N2 gas, although other purified gas, e.g., particulate free atmosphere, could be used. The purge gas can be delivered, e.g., for steps 304 and Attorney Docket No 44021362WO01; 05542-1556WO1 308, by closing valves 110a and 114a and opening valve 112a. Alternatively, valve 110a can remain open (or the system can be implemented without valve 110a), but valve 172a remains closed. In some implementations, the purge liquid for steps 304 and 308 can be delivered by opening valve 114a and closing valves 112a and 110a; the purge liquid, e.g., DI water, will flow through the delivery line 118 into the manifold. In this case, the process 300 can be performed intermittently between polishing operations, e.g., once every fifty to three hundred substrates, and can be performed without putting the polishing system in an idle mode. However, in some implementations, the purge liquid is delivered for steps 302 and 306 from the heating medium source 170 and through the boiler 110. For such an operation the valves 114a, 112a are closed and valve 110a is opened. The heating element 162 is disabled, e.g., the resistive heater is turned off, and valve 170a is opened and the liquid, e.g., DI water, is pumped into the boiler 110 so that the canister 160 is filled and the heating liquid is forced into and through the delivery line 118 into the manifold in the dispensing arm 120. In this case, because the boiler is disabled, the process 300 can be performed intermittently, e.g., when the polishing system is put into an idle mode, e.g., about once per day. For example, the polishing system may be put into idle mode for other maintenance, or when switching to polishing of a different batch of substrate. Once the purging operation is complete, the valve 170a is closed and valve 172a is opened to permit liquid in the fluid delivery line 118 and the canister 160 to drain out. Valve 112a may be open to provide back pressure to fluid in the delivery line 118. In some implementations, the liquid level is lowered until the canister 160 is partially filled (310). However, in some implementations, valve 172a remains open until liquid is entirely drained out the boiler (312), i.e., the canister 160 is empty. In this case, the boiler is then refilled (314), e.g., by opening valve 170a and closing valves 172a and 112a. Optionally, to additionally purge the canister 160, the step 312 of draining the boiler and step 314 of refilling the boiler can be repeated, e.g., two to ten times, e.g., three or four times. This can help prevent scaling build up inside the canister 160. Where multiple refill steps are performed, all except the final refill step can completely fill the canister Attorney Docket No 44021362WO01; 05542-1556WO1 160. For the final refill step, the canister should be partially refilled. Then the heating element 162 can be turned back on to resume generation of the steam. Terms of relative positioning are used to refer to relative positioning within the system; it should be understood that the polishing surface and substrate can be held in a vertical orientation or some other orientation during the polishing operation. Functional operations of the controller 90 can be implemented using one or more computer program products, i.e., one or more computer programs tangibly embodied in a non-transitory computer readable storage media, for execution by, or to control the operation of, data processing apparatus, e.g., a programmable processor, a computer, or multiple processors or computers. A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, although heating fluids are described above, the arm of the cooling system can be configured similarly, but with a coolant flowing through the arm rather than a heated fluid. Similar advantages apply if the cooling system has an arm 140 with a similar physical structure. For example, the radial profile of the mass flow rate of the coolant can compensate for temperature non- uniformities, in this case by reducing the temperature rather than increasing the temperature. Accordingly, other embodiments are within the scope of the following claims.