METHOD OF TEMPERATURE CONTROL OF A WAFER ON A CHUCK

Abstract: A temperature controlling system is disclosed which is particularly suitably usable in a step-and-­ repeat type exposure apparatus for manufacture of minute semiconductor circuit devices. The system is arranged to execute the temperature control to prevent occurrence of any local distortion of a workpiece such as a wafer due to irradiation with a radiation energy. Also, the system is effective to avoid any adverse effect due to vibration which might otherwise be caused as a result of the temperature control.

Description: This invention concerns a method of temperature control for use in a step-and-repeat type exposure apparatus. Such apparatus is used in the manufacture of semiconductor microcircuit devices. In the manufacture of semiconductor microcircuit devices, a semiconductor workpiece called a wafer is used. Since such a wafer is formed by a very thin material, it is very difficult to maintain the shape of a wafer constant during the semiconductor microcircuit manufacturing processes. This is because the shape of a wafer can be easily deformed due to a temperature change or otherwise around the wafer. Also, it is very difficult to control the temperature during the manufacturing processes to maintain the same exactly constant. In a circuit pattern printing process using an exposure apparatus, usually the printing is repeated to print different patterns on a wafer superimposingly. Any change in the shape of a wafer in the course of such a printing process causes deterioration of the pattern overlay accuracy, which might result in a failure of production of a desired circuit pattern. Particularly, in recent submicron or half micron linewidth pattern printing, such a problem, namely, a pattern overlay error, can not be disregarded. While many proposals have been made in an attempt to solving the problem of wafer deformation, one representative method is to control the temperature of a wafer or mask as a whole to thereby prevent deformation of the wafer or mask due to any temperature change. An alternative method is to control the temperature so as to positively deform a wafer or mask into a desired shape. It has also been known to control the temperature of the mask, the wafer, or wafer chuck, using a large number of individually controllable fluid jets. See for example German Patent Application No. 3213239. As disclosed therein the current temperature or temperature distribution of the specimen workpiece is measured and is used as data for controlling the temperature. It is however a disadvantage of this approach that a time lag can occur between changes of temperature and the application of corrective control. SUMMARY OF THE INVENTION On the other hand, step-and-repeat type exposure apparatuses (each called a "stepper") have been prevalently used, in the recent trend of further miniaturization of a circuit pattern and enlargement of a diameter of a wafer. In this type of exposure apparatuses, a radiation energy is locally projected upon a wafer at a time. The inventor of the subject application has found that such local irradiation of a wafer with a radiation energy (such as a light) involves a possibility of creation of local distortion of the wafer. The possibility is significantly high particularly when the intensity of the radiation energy is high. The local distortion of a wafer, if any, can easily cause an alignment error. Such an alignment error may be particularly notable, where the mask-to-wafer alignment operation should be made each time one shot area on the wafer is moved stepwise to an exposure station. As a matter of course, the alignment error results in a pattern overlay error. In an attempt to solving the above-described problem, it may be considered to use a conventional temperature controlling method. For example, it may be considered to circulate a constant-temperature gas or liquid around a wafer so as to maintain a constant temperature throughout the wafer as a whole. Depending on the manner of circulation, it may be possible to remove the energy concentrated locally upon a wafer. However, this method requires use of a large rate of flow of a temperature controlled gas or liquid, which inevitably results in an increased resistance of a passageway to the flowing temperature-controlled gas or liquid. This is undesirable because such an increased resistance easy causes an increased vibration. If such a vibration occurs during the exposure, it causes inconveniences such as uneven exposure, blurring or otherwise. If, on the other hand, the vibration occurs during the alignment operation, it causes an alignment error or prolongation of the time required for completing the alignment. Thus, the problem of such vibration can not be disregarded, particularly in the field of fine pattern printing as described hereinbefore. In accordance with the present invention there is provided a method of temperature control for use in a step-and-repeat type exposure apparatus that includes a radiation source for projecting a radiation beam onto a portion of a semiconductor wafer placed on a holding chuck having an attracting surface and a plurality of temperature control elements, said method comprising the steps of: placing a test wafer on the attracting surface of the holding chuck so that the test wafer is attracted to and held by the holding chuck; moving the holding chuck stepwise so that different portions of the test wafer are sequentially brought to an exposure position, and exposing that portion of the test wafer placed at the exposure position to a radiation beam emitted from the radiation source; each time a portion of the test wafer is exposed at the exposure position, detecting the temperature changes of said portion of the test wafer; storing data related to the detected temperature changes; removing the test wafer from the holding chuck and placing a specimen wafer on the attracting surface of the holding chuck; moving the holding chuck, bearing the specimen wafer, stepwise so that the said different portions of the specimen wafer are sequentially brought to the exposure position for exposure to the radiation beam emitted from the radiation source; and for each respective exposed portion of said specimen wafer for each exposure, controlling at least one of said temperature control elements in said holding chuck in dependance on the stored data in order to control the temperature of said portion of said specimen wafer in order to prevent distortion of said specimen wafer. In both cases disruptive vibration effects, which can be caused by single jet cooling, are obviated. By such control it is possible to prevent occurrence of distortion of the specimen wafer which may be very thin, which distortion arises from a local concentration of radiated energy. In the embodiments described below many fluid passageways (jets) are utilised and are controlled individually. Alternatively Peltier effect thermocouple cooling can be applied. In both cases disruptive vibration effects, which can be caused by single jet cooling, are obviated. Advantageously temperature control can be applied in advance of exposure for each exposure portion of the specimen wafer. It can be applied thus whilst a preceding exposure portion is under exposure. In this way alignment procedures can be applied immediately following each stepwise movement. This saves delay and contributes to an increased throughput. As a refinement, measurements may be made and stored for several exposure portions of the test wafer for each exposure and can be used to control temperature simultaneously for each current and next exposed portions of the specimen wafer. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic and diagrammatic view of a step-and-repeat type exposure apparatus; Figure 2A is a plan view of a wafer chuck table with a temperature adjusting function, usable in the apparatus of Figure 1; Figure 2B is a representation, partly in section taken on a line B-B in Figure 2A and partly in a diagram, showing details of the wafer chuck table of Figure 2A; Figure 2C is a section taken on a line C-C in Figure 2B; Figures 3A and 3B show, in combination, the flow of sequential operations made in using the apparatus of Figure 1; Figures 4A and 4B are representations for explaining the temperature control applied in the apparatus of Figure 1; Figure 5 is a plan view of a wafer, for explaining an alternative mode of using the apparatus of Figure 1; and Figure 6 is a sectional view showing the structure of a wafer chuck table with alternative temperature control. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to Figure 1 there is shown a general arrangement of a step-and-repeat type exposure apparatus. This apparatus includes a radiation energy source 1 which may be an X-ray source, an ultraviolet ray source or otherwise; a radiation source driver 2 for actuating the radiation energy source; a controller 3 for controlling the driver 2; a mask stage 12 for holding thereon a mask 11; a mask stage drive 4 for controllably moving the mask stage 12; a wafer stage 9 for movably supporting a wafer 10; a wafer stage drive 5 for controllably moving the wafer stage 9; a wafer chuck table 7 having a temperature adjusting function, for holding a wafer 10 by attraction, the table 7 being supported by the wafer stage 9; a temperature adjusting means 6 cooperable with the chuck table 7 to execute the temperature control of a wafer 10 held by the chuck table; and a central processing unit (CPU) 8 which is electrically coupled to each of the mask stage drive 4, the wafer stage drive 5, the temperature adjusting means 6 and the controller 3, for executing various controls to be made in the exposure apparatus as a whole. A mask 11 placed on the mask stage 12 has a pattern which is transferred, when the mask is irradiated with a radiation energy supplied from the radiation source 1, onto different shot areas on a wafer 10, in a predetermined order, with the cooperation of the wafer stage 9. In this step-and-repeat pattern transfer process, temperature control (controlled cooling) is locally applied to each portion of the wafer 10 when it is at the exposure position, by using the attracting table 7 having a temperature adjusting function. Details of this will now be explained with reference to Figures 2A - 2C. Of these Figures, Figure 2A is a top plan view, Figure 2B is a schematic view partly in section along a line B-B in Figure 2A, and Figure 2C is a section along a line C-C in Figure 2B. As best seen in Figure 2B, the attracting table 7 includes a base member 21 which is made of an organic material or ceramics, a pair of embedded electrodes 22, a plurality of passageways 23 each for allowing flow of a temperature adjusted water or gas, and an electrically insulative layer 24. The temperature adjusting means 6 (Figure 1) comprises a plurality of temperature controls 6a each for controlling the flow rate and the temperature of the fluid flowing through a corresponding passageway 23, and a fluid supply source 6b for supplying a fluid into each of the passageways 23. Denoted at 28 is a heat insulating material and, at 29, a plurality of temperature sensors. The fluid supply source 6b may be of the type that it can supply a temperature controlled fluid and thus each control 6a may be one that simply controls the flow rate of the fluid. As seen from Figure 2B, a wafer is attracted to the wafer chuck table by an electrostatically attracting force. However, use of such an electrostatic chuck is not indispensable, and a vacuum attraction process, a magnetically attracting process or otherwise may be used. The sensors 29 may be provided in the base member of the chuck table at those positions which correspond respectively to the sites which should be brought to the exposure position in sequence during the step-and-repeat exposures of a wafer. Of course, the positions of the sensors 29 may be determined, while taking into account the possibility of changing the pitch of stepwise movement (stepwise feeding). Preferably, the passageways 23 are provided in a one-to-one relationship with the temperature sensors 29, as best seen in Figure 2B. More specifically, these passageways 23 have fluid discharging ports which are in a one-to-one relationship with the sensors 29. On the other hand, the chuck table is formed with four outlet ports through which the fluid can be exhausted. However, one outlet port may be provided in association with one fluid discharging port of one passageway. Also, one outlet port may be provided for two fluid discharging ports of two passageways. The temperature control is made by flowing through a selected passageway a fluid, supplied from the fluid supply source 6b, with the temperature and the flow rate of the flowing fluid being controlled by a corresponding adjusting device 6a under the control of the CPU 8. Details of the temperature control will now be explained, particularly with reference to Figures 3A and 3B. First, in the alignment and exposure sequence, a desired radiation energy (intensity of irradiation) and a desired irradiation time (exposure time) for each step position on a wafer (the site of each portion of the wafer to be placed at the exposure position), as well as the order of exposures of these step positions on the wafer, are set (step S1). Also, a desired temperature of the wafer to be maintained is set. Subsequently, as a sample, a test wafer is placed on the chucking table and, then, an electric voltage is applied to the electrodes 22. As a result, the test wafer is electrostatically attracted to and held by the insulative layer 24 (step S2). After this, the test wafer is subjected to step-and-repeat exposures in accordance with the predetermined order (steps S3 - S7 and S13). Along the execution of the exposures, any local temperature change at each step position on the test wafer is detected by a corresponding temperature sensor 29 (step S8). Simultaneously with the detection of temperature change, the selection of an appropriate passageway or passageways (if desired) and the control of the flow rate and temperature of the fluid to be flown through the selected passageway, are made so as to correct the detected temperature change to thereby retain the desired wafer temperature (steps S9 and S10). Those data concerning the detected temperature changes as well as those data concerning the selection of passageways and the controlled flow rate and temperature of the fluid, in respect to every step position on the wafer, are stored into a suitable memory (not shown) of the CPU 8 (step S11). Those data are used during an actual photoprinting process to control the temperature and the flow rate of a fluid flowing through a selected passageway 23 or passageways (if desired), under the influence of the CPU 8 and with the cooperation of a corresponding adjusting device 6a, to thereby execute concentrated temperature control (controlled cooling) of each step position of a wafer and a portion thereof adjacent to that step position. By this, the entire wafer can be maintained substantially at the set temperature and, therefore, any local distortion of the wafer can be prevented. In this example the temperature control (controlled cooling) is made by controlling the flow rate in the manner such as depicted in Figure 4B, to a temperature change such as depicted in Figure 4A, wherein Figure 4A exemplifies a temperature change at the time of exposure of a certain step position on a wafer, while Figure 4B depicts a necessary flow rate control for the correction of the temperature change in Figure 4A. The necessary flow rate is determined in the CPU 8, while taking into account the throughput of the apparatus. After the collection of the temperature control data by using a test wafer is completed, the test wafer is replaced by a wafer which is to be processed actually (step S14), and the step-and-repeat exposures are effected (steps S15 - S19 and S23). Then, as the step positions of the wafer are brought to the exposure position in sequence during the step-and-repeat exposure process, the flow rate of a fluid flowing through a corresponding passageway 23 is controlled in accordance with the local temperature change at a corresponding step position, the control being made in a timed relationship with the irradiation time based on the exposure sequence, under the control of the CPU 8 in which the data have been stored. By this, any local temperature increase at each step position of the wafer due to the irradiation with the exposure energy can be corrected. Thus, by controlling the temperature of the chucking table locally, the temperature of each portion of the wafer can be controlled locally (steps S20 - S22). In this example the temperature control is made by using a fluid flowing through a passageway positioned beneath a step position when the same is at the exposure position and, if desired, a fluid flowing through one or more passageways adjacent thereto. The CPU 8 is operable, as the portions of the wafer are exposed in the step-and-repeat manner, to start the temperature control to each step position before the temperature thereat increases due to the irradiation with the radiation energy and, thus, the CPU 8 controls the flow rate of the fluid to assure this. After exposure at a step position is completed, the temperature control thereto is finished, and the temperature control to a next step position is made in a similar manner. Since, however, the next step position is predetermined in accordance with the predetermined exposure sequence, the temperature control to the next step position may be made before the completion of the exposure at the current step position. In that case, it is possible to prevent accumulation of heat at the next step position which easily causes local distortion of the wafer and, thus, an alignment error at the next step position. Therefore, by doing so, the alignment operation at the next step position can be initiated just after the stepwise movement to be made after the completion of the exposure at the current step position. Figure 5 shows a second embodiment wherein the manner of temperature detection is different. In Figure 5, reference numeral 10 denotes a wafer and reference numeral 51 denotes each step position (shot area). While in the foregoing embodiment, the temperature measurement is made at each step position, in this embodiment, the temperature measurement is made at selected ones of the plural step positions. More specifically, only with regard to desired ones of the step positions, the temperature change of the wafer portions at the selected step positions and portions in the neighborhood thereof is measured. In Figure 5, for example, only a shot area A may be selected at the measurement step position, and the measurement may be made to the temperature change in the shot area A as it is irradiated with the radiation energy and, at the same time, the measurement may be made to the shot areas B and C adjacent thereto. By using data obtained therefrom, the amount of temperature control at every shot position of the wafer is determined. This can be made in a case where the exposure time and the exposure intensity are substantially constant with regard to the different shot positions. As described, the temperature measurement is made in this embodiment not only to a certain selected step position (shot area) but also to one or more (two in this embodiment) step positions in the neighborhood thereof. This is done to allow temperature control also to those step positions so as to remove any heat propagated thereto from the current step position, the control being made before each of those second and third step positions is brought to the exposure station. Namely, the temperature control to those neighboring step positions is made at the same time as the temperature control to the current step position. In other words, in the present embodiment, by using those data obtained by the temperature measurement made to plural neighboring step positions including a current step position, the magnitude of temperature control to be made to the current step position as well as the next step position and the one following the step, is determined. The control is then made in accordance with the determined data. This allows good temperature control in a stepper. Figure 6 shows a third embodiment of the present invention. Same reference numerals, like those of the foregoing embodiments, are assigned to similar or corresponding elements. In Figure 6, denoted at 61 are N-type elements; at 62, P-type elements; at 63, a radiating fin; at 64, a heat absorbing fin. The elements 61 and 62 are formed by an intermetallic compound such as, for example, Bi₂Te₃ and provide electronic cooling members of a type in which the temperature at a junction decreases in response to the application of an electric current, due to the Peltier effect. Like the foregoing embodiments, the heat absorbing fin 62 have elements which are disposed to allow concentrated temperature control at each step position on a wafer during the step-and-repeat exposures thereof. When, in operation, electric currents are supplied from an unshown electric power source to the P-type and N-type electronic cooling members 61 and 62, the bottom surface of the chuck base member 21 is cooled due to the Peltier effect. Of course, the supply of electric current is controlled in response to the instruction signals from a CPU, so that the temperature at each step position on the wafer is controlled and maintained at a desired wafer temperature to be retained. By this, substantially the same effects are obtainable.

Claims: A method of temperature control for use in a step-and-repeat type exposure apparatus that includes a radiation source for projecting a radiation beam onto a portion of a semiconductor wafer placed on a holding chuck having an attracting surface and a plurality of temperature control elements, said method comprising the steps of: placing a test wafer on the attracting surface of the holding chuck so that the test wafer is attracted to and held by the holding chuck; moving the holding chuck stepwise so that different portions of the test wafer are sequentially brought to an exposure position, and exposing that portion of the test wafer placed at the exposure position to a radiation beam emitted from the radiation source; each time a portion of the test wafer is exposed at the exposure position, detecting the temperature changes of said portion of the test wafer; storing data related to the detected temperature changes; removing the test wafer from the holding chuck and placing a specimen wafer on the attracting surface of the holding chuck; moving the holding chuck, bearing the specimen wafer, stepwise so that the said different portions of the specimen wafer are sequentially brought to the exposure position for exposure to the radiation beam emitted from the radiation source; and for each respective exposed portion of said specimen wafer for each exposure, controlling at least one of said temperature control elements in said holding chuck in dependance on the stored data in order to control the temperature of said portion of said specimen wafer in order to prevent distortion of said specimen wafer. A method as claimed in claim 1 wherein said step of detecting the temperature changes of said portion of the test wafer is performed indirectly, said step being performed by detecting local temperature changes of the attracting surface of the holding chuck. A method as claimed in either of claims 1 or 2 wherein said step of controlling said at least one of said temperature control elements includes controlling the flow rate and temperature of a fluid which is caused to flow through a respective passageway positioned beneath each respective exposure portion of said specimen wafer. A method as claimed in any preceding claim wherein: to control the temperature of the specimen wafer, temperature control is applied locally to the holding chuck, for controlling the temperature of the attracting surface thereof for the respective next exposed portion of the specimen wafer and is commenced before completion of exposure of each currently exposed portion thereof; and alignment operation for each next exposure is initiated immediately following completion of each respective current exposure. A method as claimed in any preceding claim wherein: for each exposure of said test wafer temperature changes are detected not only for each respective exposed portion but also for one or more neighbouring portions; data, relating to each of the one or more detected temperature changes, is stored; and to control the temperature of the specimen wafer, temperature control is applied locally to the holding chuck for controlling the temperature of the attracting surface thereof for all of said one or more neighbouring portions before each is moved to said exposure position and at the same time as temperature control is applied to each respective exposed portion.

Inventors: FUJIOKA, HIDEHIKO
Applicants: CANON KABUSHIKI KAISHA
Application year: 1988
Main classification: H01L21
IPC codes: H01L21, G03F7