Method and apparatus for polishing

Abstract: A polishing apparatus can improve the uniformity of thickness within a workpiece or reduce thickness variation between serially polished workpieces. The polishing apparatus comprises a polishing unit having a polishing tool for providing a polishing surface and a workpiece holding device for holding the workpiece. A polishing solution supplying device is provided for supplying a polishing solution into a polishing interface between the surface of the workpiece and the polishing surface. A temperature of the polishing interface is controlled according to at least an ambient temperature of a polishing space surrounding the polishing unit, as a variable parameter.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates in general to a polishing apparatus, and relates in particular to a polishing apparatus for precision polishing of a workpiece such as semiconductor wafer to a flat and mirror finish. Description of the Related Art As semiconductor devices become more highly integrated, wiring spacing becomes finer and interline spacing also becomes very narrow. Photolithographic reproduction of circuits using stepper printing process for such highly integrated devices requires the depth of focus be very shallow so that the wafer surface must be extremely flat. A known method of producing a flat surface on semiconductor wafer is to press the wafer held on a holding device against a polishing tool such as, for example, polishing table with a polishing cloth mounted thereon, and supply a polishing solution to the interface while sliding the tool relative to the wafer. This chemical-mechanical polishing process employs, in addition to mechanical polishing process using abrasive grains, a chemical polishing process using acidic or alkaline solutions. In such an apparatus, it is known that the removal rate of the surface material (polishing speed) is dependent not only on the force pressing the wafer against the polishing tool (polishing cloth) and sliding speed, but also temperature of the polishing interface (polishing temperature). It is important to be able to control the polishing speed constant, not only for producing high flatness but also for determining correctly an endpoint of a polishing process. Therefore, wafer quality control, such as flatness control within a wafer, control of variation in flatness from a wafer to a wafer, and endpoint determination are all related to the ability to control the polishing interface at a desired temperature. Consistency in the flatness from wafer to wafer is particularly important in improving the reproducibility of the quality of wafers produced in a serial polishing process. Further, the polishing interface temperature is a particularly crucial element when polishing a semiconductor wafer provided with different types of thin films provided on the same surface. Such combination includes oxide film and nitride film combination, and combination of metal film such as tungsten or titanium and barrier layer such as titanium nitride or oxide. It is anticipated that the polishing interface temperature is governed by a heat balance between heat generated by frictional forces of polishing, heat generated or removed by a polishing solution supplied to the polishing interface, and heat conduction through the wafer holding section and polishing tools. Therefore, current practices are to control the polishing solution temperature and to use a polishing table having internal cooling passages to keep the polishing table at a constant temperature. However, it has been difficult to maintain the polishing interface temperature at a target temperature by relying only on controlling the polishing solution and polishing table temperatures of a polishing apparatus. Polishing performance at inconsistent temperatures has been a cause of wafer-to-wafer variation in thickness of films or non-uniformity of flatness within a wafer. Such a variation in wafer thickness also makes it difficult to determine an endpoint of polishing based on a target thickness. Attention is drawn to, US-A-5 643 060 which shows a method and an apparatus for polishing a workpiece. Said apparatus comprising a rotatable platen assembly having a polishing pad thereon for defining a polishing surface and a polishing head for supporting the wafer and pressing the wafer against the polishing pad. Said apparatus further comprising a slurry supply system for introducing a chemical slurry into the polishing environment to facilitate polishing of the workpiece. Further, a controller for controlling operational parameters, such as a temperature at the wafer surface is provided. SUMMARY OF THE INVENTION It is an object of the present invention to provide an apparatus and a method capable of improving the uniformity of thickness within a workpiece or reducing thickness variation between serially polished workpieces, and to facilitate detection of an endpoint of polishing. In accordance with the present invention, an apparatus and a method as set forth in claims 1 and 8 is provided. Preferred embodiments of the invention are claimed in the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is an overall cross sectional view of the polishing apparatus of the present invention; Figure 2 is a flowchart for control methodology of the present invention; and Figure 3 is a flowchart for another control methodology of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS In the following, an embodiment will be presented with reference to Figure 1. The polishing apparatus comprises a polishing unit comprised by a polishing table (polishing tool) 12 having a polishing cloth 10 mounted on its top surface and a top ring 14 for holding a wafer on its bottom surface to press against the polishing cloth 10, and a supply nozzle 16 to supply a polishing solution Q on the polishing cloth 10. The surface of the polishing cloth 10 constitutes a polishing surface. The apparatus is provided with driving devices not shown in the drawing for independently rotating the polishing table 12 and the top ring 14, and a pressing device (air cylinder, for example) to press the top ring 14 onto the polishing table 12. The polishing solution Q is supplied from a polishing solution supply system through a solution delivery pipe 18 to be supplied through the supply nozzle 16. A temperature sensor 20 for measuring the solution temperature and a solution temperature adjusting device 22 for heating or cooling the polishing solution Q are provided in selected locations of the polishing solution supply system. The polishing table 12 is provided with an internal medium space 24, for flowing a thermal medium (cooling/heating) C, communicating with an external thermal medium supply pipe 28 of a thermal medium supply system through a thermal medium passage formed in a rotation shaft 26 which supports the polishing table 12. The thermal medium supply system is also provided with a medium temperature adjusting device 32 having a temperature sensor 30 for detecting the temperature of the thermal medium C. Output data from the temperature sensors 20, 30 are input into a main controller 34 which outputs control signals to the temperature adjusting devices 22, 32. The polishing apparatus is housed in an enclosed polishing room (polishing space) 38 formed by dividing walls 36 to separate the polishing room 38 from the surrounding spaces The polishing apparatus is provided with an air conditioning apparatus 42 which draws outdoor air and passes it through a filter section to remove dust particles and, after adjusting the temperature and humidity, supplies the conditioned air to the polishing room by uniformly dispersing the air through the duct openings 40 provided at the upper section of the polishing room 38. An exhaust pump 46 is provided to exhaust the air including mist and abrasive particles through exhaust duct openings 44 provided in the bottom section of the polishing room 38. These arrangements produce consistent down drafts of clean air within the polishing room 38, and are effective in preventing contamination of polished wafers by preventing mist or solid particles generated by polishing from swirling upwards. In this case, the exhaust air is discharged to outdoors, but a part or all of the exhaust air may be re-circulated, or, in some cases, subjected to heat exchange between incoming and exhausting air to improve power conservation efficiency. In this embodiment, the inside pressure of the polishing room 38 is adjusted so as to be less than its neighboring units within the polishing facility, such as the cleaning unit in which polished wafers are cleaned, or the wafer storage unit in which polished wafers or wafers to be polished are stored. This is because the polishing room 38 has the lowest cleanliness index within the polishing facility, so that the pressure differential is aimed at preventing contaminants from flowing out of the polishing room into the adjoining spaces. A temperature sensor 48 is provided in certain locations in the polishing room 38 to control the ambient temperature in conjunction with the main controller 34, which controls the temperature of the supply air from the air conditioning apparatus 42 in accordance with an ambient temperature measured by the sensor 48 and a selected polishing temperature To. The temperature sensor 48 should detect a representative temperature of the ambient air in the polishing room 38, and it is preferable to take an average value of temperatures from several sensors placed around the room, as necessary. Although one controller 34 is shared by control temperature adjusting devices 22, 32 and air conditioning apparatus 42, but it is also permissible to provide a dedicated controller and sensors to each device, and an overall control may be assigned to a separate controller or one of those local controllers. In the following, the control methodology of the polishing temperature using the polishing apparatus will be explained. Control methodology is based on controlling the polishing temperature by considering the temperature of the polishing space 38 in addition to those of the polishing solution Q and thermal medium C. "Polishing temperature" refers to the temperature at the polishing interface between the surface of the wafer being polished and the polishing surface on the polishing table or the polishing cloth mounted on top of the polishing table. Control methodology may be based on a number of parameters and objects for control and their combinations, and some specific examples will be presented. Figure 2 shows a first control methodology for controlling the polishing process so that polishing is commenced after confirming that a target temperature has been reached substantially in the polishing room 38, polishing solution Q and thermal medium C. In step 1, a target polishing temperature To is input into a computer (controller) 34. Target polishing temperature To is pre-determined through results obtained by trial polishing employing a suitable processing conditions such as the nature of the workpiece to be polished, polishing tools, polishing solution and polishing speed. Polishing solution Q and thermal medium C may be pre-heated to a temperature near the target temperature. In step 2, temperature sensors 48, 30 and 20 detect the ambient temperature Tr, solution temperature Tq and thermal medium temperature Tc within the polishing table 12, respectively. In step 3, controller 34 examines whether the measured temperatures are within an allowable range of the target temperature. If one or more operating temperatures are out of the allowable range, then in step 4, the controller 34 activates the temperature adjusting device 22, 32 or air conditioning apparatus 42 to adjust the operating temperature(s), and returning to step 2, the sensors are asked to re-check the temperatures. When all the operating temperatures are within or have reached the allowable range, then in step 5, polishing is started. After starting polishing, the program returns to step 2 to re-measure the operating temperatures, and the control action is repeated by following either of the two loops, steps 2, 3, 4 →2 or steps 2, 3, 5→2 to constantly maintain all the temperatures within the allowable range of the target polishing temperature To. According to this methodology, if the target polishing temperature To is selected at 16 °C and the allowable range to 2 °C, the temperatures of polishing solution Q, thermal medium C for the polishing table 12 and polishing space 38 are all maintained at 16 ± 2 °C. Therefore, heat transfer to the space 38 is essentially eliminated and the temperature of the surface being polished can be maintained precisely, thereby producing uniformly flat surface on the wafer and appropriate removal rate of the surface material determined by the nature of the workpiece to be polished. Table below shows the results of trials to compare the polishing temperatures, i.e., surface temperatures of cloth 10, during the method according to the invention where temperature control over polishing solution Q, thermal medium C and polishing room 38 is conducted, with that during a conventional method where temperature control only over polishing solution Q and thermal medium C is conducted. Case 1 Case 2 Conventional Polishing Solution 15 °C 40 °C 15 °C Thermal Medium 15 40 15 Polishing Room 16 39 23 Polishing Temperature 17 39 23 Target Temperature 16 40 16 These results showed that the polishing temperature can be controlled precisely by controlling the temperatures of not only the polishing solution Q and thermal medium C for the polishing table 12 but that of the polishing room 38. The temperature of polishing room 38 can be controlled by controlling the air temperature using a normal air conditioner 42 in an arrangement shown in Figure 1. The first method described above is based on controlling individual temperatures separately for solution Q, thermal medium C and polishing room 38 so that the method itself is simple and highly practical. In this example, alarming or stopping provision for alerting an excessive deviation in any of the temperatures may be provided, if necessary. A second methodology for polishing temperature control will be explained in the following. In the second method, the ambient temperature is not the object of control but it is used as a guiding parameter to control the temperatures of other parameters such as the polishing solution Q and thermal medium C of the polishing table 12. Specifically, when the temperature of the polishing space 38 is higher than the target polishing temperature To, the polishing temperature tends to become higher than that when the temperature of the polishing space 38 is To. Under such conditions, the solution and medium temperatures are maintained lower to compensate for the temperature rises which would be caused by the higher temperature in the polishing space 38. This method is appropriate when the polishing apparatus is located not in a divided enclosure such as the one shown Figure 1, but is located in a larger space such as normal undivided clean room or other large space where it is not desirable to change the polishing ambient temperature by considering only the requirements of the polishing apparatus, or when the air density in the polishing space is so low that the ambient temperature control is difficult. Figure 3 shows a flowchart for the second methodology. In step 1, a target polishing temperature To is input into a computer (controller) 34, and in step 2, temperature sensors 48, 30, 20 detect the ambient temperature Tr, solution temperature Tq and thermal medium temperature Tc. In step 3, computer 34 calculates target temperatures Tqo and Tco, respectively, for the solution temperature Tq and the medium temperature Tc based on the measured temperatures, according to the following equations, for example: Tqo = To - k 1 DT, Tco = To - k 2 DT where DT = Tr - To, and k 1 , k 2 are experimentally determined constants. In step 4, measured temperatures Tq, Tc are compared with the target temperatures Tqo, Tco, and if the operating temperatures are not within the allowable range, the program proceeds to step 5 to adjust the operating temperatures by activating the respective adjusting devices 22, 32, and returns to step S2 to re-check the current temperatures. When all the operating temperatures are at or have been adjusted to the allowable range, polishing is started in step 6. After polishing is started, the program returns to step 2 to re-check the temperatures, and the control action is repeated by following either of the two loops, steps 2, 3, 4, 5→2 or steps 2, 3, 4, 6→2 so that the temperatures Tq and Tc are maintained within the allowable range. According to this methodology, if the target polishing temperature To is selected at 16 °C and the allowable range to 2 °C, the temperatures of polishing solution Q, thermal medium C for the polishing table 12 are controlled so as to compensate for the temperature differences between the polishing space temperature Tr and the target polishing temperature To. Thus, the temperature of the surface being polished will be maintained at the target polishing temperature To precisely, thereby producing uniform removal rate determined suitable for the workpiece to be polished and a uniform flatness within a workpiece surface. Table below shows the results of polishing temperature measurements according to the second methodology. Case 3 Case 4 Conventional Polishing Solution 10 °c 48 °C 15 °C Thermal Medium 10 47 15 Polishing Room 23 23 23 Polishing Temperature 17 39 23 Target Temperature 16 40 16 In this embodiment, ambient temperature of the polishing room 38 was used only as a guiding parameter and was not controlled, but it is obvious that the ambient temperature can be controlled within a range compatible with other required conditions. Also, method may be based on measuring the polishing temperature, i.e., cloth temperature of the cloth 10 or the object temperature of workpiece W, by suitable means and adjust the control parameters including ambient temperature according to the measured results. Such temperature sensors may be selected from the available sensor types such as sensors embedded in polishing table 12 or top ring 14 or remote radiation type such as infrared sensors which can measure the cloth temperature in a non-contact manner.

Claims: A polishing apparatus for polishing a surface on a workpiece (W), said polishing apparatus comprising: a polishing unit having a polishing tool (12) for providing a polishing surface and a workpiece holding device (14) for holding said workpiece (W); a polishing solution supplying device (16) for supplying a polishing solution (Q) into a polishing interface between said surface of said workpiece (W) and said polishing surface; an ambient temperature sensor (48) for directly or indirectly detecting an ambient temperature (Tr) of a polishing space surrounding said polishing unit (12); and a polishing temperature control device (34) for controlling temperatures of said polishing solution (Q) and a thermal medium (C) flowing in an internal passage in said polishing tool (12) according to detected results of said ambient temperature sensor (48) to maintain a polishing temperature at said polishing interface. The polishing apparatus according to claim 1, wherein said apparatus is provided with an ambient temperature control device (34) to control said polishing space at a specific temperature. The polishing apparatus according to claim 1 or 2 , wherein said apparatus is provided with a sensor device to detect a polishing surface temperature. The polishing apparatus according to any one of claims 1 to 3, wherein said polishing temperature control device (34) maintains air temperature of air being supplied to said polishing space in a form of a downflow. The polishing apparatus according to any one of the preceding claims, having means for inputting a target polishing temperature (To) into said polishing temperature control device (34). The polishing apparatus according to claim 5, wherein said polishing temperature control device (34) controls temperatures of said polishing solution (Q) and said thermal medium (C) so as to be said target polishing temperature (To). The polishing apparatus according to claim 5, wherein said polishing temperature control device (34) controls temperatures of said polishing solution (Q) and said thermal medium (C) so as to be a lower temperature than said target polishing temperature (To) when said ambient temperature (Tr) is higher than said target temperature. A method for controlling a surface temperature of a workpiece (W) by a polishing apparatus, said apparatus comprising a polishing unit having a polishing tool (12) for providing a polishing surface and a workpiece holding device (14) for holding said workpiece (W), an ambient temperature sensor (48) for detecting an ambient temperature (Tr) of a polishing space surrounding said polishing unit, and a polishing solution supplying device (16) for supplying a polishing solution (Q) into a polishing interface between said surface of said workpiece (W) and said polishing surface, wherein temperatures of said polishing solution (Q) and a thermal medium (C) flowing internally in said polishing tool (12) are controlled according to a temperature in a polishing space surrounding said polishing unit to maintain a polishing temperature at said polishing interface. The method according to claim 8, wherein a target polishing temperature (To) at said polishing interface is inputted for maintaining said polishing temperature at said polishing interface. The method according to claim 9, wherein temperatures of said polishing solution (Q) and said thermal medium (C) are controlled so as to be said target polishing temperature (To). The method according to claim 9, wherein temperatures of said polishing solution (Q) and said thermal medium (C) are controlled so as to be a lower temperature than said target polishing temperature (To) when said ambient temperature (Tr) is higher than said target temperature. The method according to any one of claims 8 to 11, wherein air temperature of air being supplied to said polishing space in a form of a downflow is maintained.

Inventors: KAWAMOTO TAKAYOSHI, KIMURA NORIO, YASUDA HOZUMI, YOSHIDA HIROSHI, KAWAMOTO, TAKAYOSHI, KIMURA, NORIO, YASUDA, HOZUMI, YOSHIDA, HIROSHI
Applicants: EBARA CORP, EBARA CORPORATION
Application year: 1998
Main classification: B24B37
IPC codes: B24B49, B24B37, H01L21