Method of holding a substrate

Abstract: A substrate holding device includes a holding table having a reduced pressure passageway; a pressure gauge for measuring a value related to the pressure in the reduced pressure passageway; a pump for producing a pressure difference between a first surface of the substrate to be attracted to the holding table and a second surface of the substrate not to be attracted to the holding table; a valve which can be opened/closed for control of the pressure in the reduced pressure passageway; a pressure control system for controlling the opening/closing of the valve on the basis of an output corresponding to the value measured by the pressure gauge; and a temperature control system for controlling the temperature of the holding table.

Description: FIELD OF THE INVENTION AND RELATED ART This invention relates to a method of holding a substrate on a holding chuck, through a difference between pressures applied respectively to a surface of the substrate, facing to the holding shuck, and another surface of the substrate remote from the holding shuck. In the field of manufacture of semiconductor microcircuits, as such a substrate holding device, a vacuum chuck is known wherein a semiconductor wafer is held through a difference between pressures applied respectively to both sides of the semiconductor wafer. Also, proposals have been made to control the temperature of a wafer chuck, holding a semiconductor wafer at an exposure position in an exposure apparatus, thereby to control the temperature of the wafer so as to prevent expansion/contraction of the wafer resulting from changes in temperature of the wafer during the exposure operation or to correct distortion of the wafer caused by any other process. An example is illustrated in Figure 5, wherein a wafer chuck 6 is arranged to hold a wafer 5 by vacuum applied thereto through an evacuation passageway 12 and wherein the wafer chuck 6 is formed with a flow passageway 7 through which temperature controlled circulating water is flown to perform temperature control of the wafer 5. In X-ray lithography for manufacture of microcircuits, a mask and a wafer are disposed close to each other with a clearance of an order of a few tens of microns. Thus, the temperature of the mask easily follows the temperature of the wafer and, therefore, temperature rise in the wafer easily produces thermal distortion of the mask. This necessitates exact temperature control of the wafer, as compared with conventional lithography using light other than X-rays. In this respect, a vacuum applying system such as shown in Figure 6 wherein vacuum is simply or uncontrollably applied to the back of the wafer is not desirable. This is because mere application of vacuum through a vacuum pump or the like substantially completely removes a gas in the inside of the wafer chuck and in the interspace between the wafer and the wafer chuck, wherein such gas serves as a heat transfer medium. This easily results in an increase in the contact thermal resistance between the wafer and the wafer chuck. In this regard, experiments were performed wherein a wafer chuck made of an SUS material was used and, by using a vacuum pump, vacuum was applied to the back of a wafer. The result is that the contact thermal resistance was increased to 10 2 deg.cm 2 /W. This means that, even if the temperature of the wafer chuck is constant, the application of energy of 100 mW/cm 2 to a wafer in the X-ray exposure, for example, causes temperature rise in the wafer by a few degrees. Since the temperature rise in the wafer causes temperature rise in the mask, there is a possibility of thermal distortion of the latter which results in positional deviation of a pattern. An example of wafer chuck which holds a wafer by vacuum attraction is disclosed in U.S. Patent No. 4 213 698. In this structure, the wafer chuck is provided with a number of pins, and a wafer is attracted toward and supported by these pins. The wafer is thus supported while good wafer flatness is maintained. However, this document does not mention to controllability of the wafer attracting pressure. An aspect of the present invention is to provide a method and device for holding a substrate in which the temperature of a substrate can be controlled precisely. It is another aspect of the present invention to provide a method and device for holding a substrate suitably applicable to an exposure apparatus for manufacture of semiconductor microcircuits, wherein a semiconductor substrate is held at an exposure position through a difference between pressures applied respectively to both sides of the substrate and wherein any temperature rise in the substrate which otherwise might be caused by the exposure operation can be reduced. It is a further aspect of the present invention to provide a method and device for holding a substrate by which, through the control of the temperature of a wafer chuck, holding a wafer, any temperature rise in a mask and a wafer which otherwise might be caused by the exposure operation can be reduced, whereby correct transfer of a pattern of the mask to the wafer is ensured. Features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings, in which:- Figure 1 is a schematic and diagrammatic view of an exposure apparatus with a substrate holding device, according to an embodiment of the present invention. Figures 2A and 2B are a graph and a schematic representation, illustrating temperatures at different sites about a wafer, as assumed during the exposure operation. Figure 3 is a graph showing the relationship between the contact thermal resistance and the pressure. Figure 4 is a schematic and diagrammatic view of an exposure apparatus with a substrate holding device, according to another embodiment of the present invention. Figure 5 is a schematic view showing the structure of a known type wafer chuck. Before explaining the operation of the present invention, the temperature rise in a mask during exposure will first be described. If in Figure 1, the back of the wafer chuck 6 is maintained at a certain temperature T 0 by means of constant temperature circulating water, at different sites x 0 - x 3 such as shown in Figure 2B, the temperatures are such as shown in Figure 2A wherein the axis of abscissa denotes the position x while the axis of ordinate denotes the temperature. As regards the position x, as seen in Figure 2B, a predetermined position on the constant temperature water side (back face) of the wafer chuck 6 is taken as an origin and, along the direction of irradiation of exposure energy (X-rays 1) to a mask 4 and a wafer 5, the interface between the wafer chuck 6 and the wafer 5 is denoted by x 1 , the front face of the wafer 5 is denoted by x 2 and the position of the mask substrate 4 is denoted by x 3 . As seen in Figure 2A, at the interface x 1 between the wafer 5 and the wafer chuck 6, there is interruption in the temperature. This is because of the presence of the contact thermal resistance between the wafer 5 and the wafer chuck 6. The temperature rise ΔT (= T 3 - T 0 ) in the mask 4 being exposed with the X-rays 1, for example, is given by the following equation: ΔT = (tc/λc + R + tw/λw + g/λg)Qt where tc is the thickness of the wafer chuck 6 with respect to the direction of irradiation, λc is the heat conductivity of the wafer chuck, tw is the thickness of the wafer 5 with respect to the direction of irradiation, λw is the heat conductivity of the wafer, g is the proximity gap between the mask 4 and the wafer 5 with respect to the direction of irradiation, λg is the heat conductivity of the gas in the proximity gap, and R is the contact thermal resistance between the wafer chuck 6 and the wafer 5. Also, Qt is the strength of the X-rays 1 (Figure 1) as projected to the mask 4 per unit time and unit area. The thermal distortion Δ l of the mask 4 resulting from the temperature rise can be expressed by using a linear expansion coefficient α and the exposure view angle 2 l (which corresponds to the range for the pattern of the mask 4 to be transferred to the wafer), in the following manner: Δ l = l ·αΔT Here, using a silicon nitride (Si 3 N 4 ) mask having a linear expansion coefficient α (α = 2.7x10 -6 [1/deg]) and a view angle 30 mm ( l = 15 mm) in a He gas ambience and under the conditions shown in Table 1, below, the thermal distortion in the exposure will now be considered. Proximity Gap g = 10 microns (µm) Wafer Chuck (material: SUS) λc = 0.245 [W/cm·deg] tc = 0.2 cm Si Wafer λw = 0.84 [W/cm·deg] tw = 0.05 cm X-ray Strength Qt = 0.1 [W/cm 2 ] First, by using Figure 3, temperature rise ΔT in an occasion where the interspace between the wafer chuck 6 and the wafer 5 is uncontrollably evacuated as in the Figure 5 example, will be determined. Figure 3 illustrates the contact thermal resistance in an occasion where a He gas or an air is confined within a small interspace between the wafer 5 and the wafer chuck 6 (substrate holding table), wherein the axis of abscissa denotes the pressure in the small interspace while the axis of ordinate denotes the contact thermal resistance. In this case, the pressure difference between both sides of the wafer 5 produced by the vacuum supplied to the wafer chuck 6 is set to be equal to 19998 Pa (150 Torr). It is seen in Figure 5 that, when the minute space between the wafer chuck and the wafer is maintained at complete vacuum, the contact thermal resistance R is equal to 10 2 Kcm 2 /W. When this value and the exposure conditions mentioned above are substituted into equation (1), then the following results: ΔT ≒ 10 (deg) Accordingly, from equation (2), the thermal distortion Δ l is 0.81 micron. It is considered that the maximum tolerable thermal distortion is 0.025 micron where the alignment precision is 0.06 micron, and the aforementioned value is extraordinarily greater than this. Next, description will be made of the principle of operation of the present invention. According to the present invention, for exposure of a mask 4 and a wafer 5, as seen in Figure 1, the gas in a reduced pressure duct 11 is drawn by a drawing pump 17, such that the pressure in the clearance between the wafer 5 and the wafer chuck 6 is reduced. This produces a pressure difference between both sides of the wafer 5, by which the wafer 5 is attracted to and held on the wafer chuck 6. In this instance, the pressure in the reduced pressure duct 11 is controlled on the basis of the measurement through a pressure gauge 13a so as to maintain the contact thermal resistance between the wafer 5 and the wafer chuck 6 at a predetermined value. Such a predetermined value for the contact thermal resistance is set to ensure that the heat in the wafer 5 is discharged to prevent temperature rise of the wafer 5 thereby to avoid temperature rise in the mask 4, such that the pattern of the mask 4 can be printed on the wafer 5 correctly. The contact thermal resistance R can be written from the thermal distortion tolerance Δ l 0 and from equations (1) and (2), as follows: R ≦ (Δ l 0)/( l ·α)·1/Qt - (tc/λc + tw/λw + g/λg) Substituting the exposure conditions set forth in Table 1 into equation (3), it follows that: R ≦ 4.6 [deg·cm 2 /W) It is seen from Figure 3 that the pressure in the clearance between the wafer 5 and the wafer chuck 6 which produces contact thermal resistance R of not greater than 4.6 (deg·cm 2 /W), is approximately not less than 6666 Pa (50 Torr). As described, in the present invention, the pressure in the space between the wafer chuck 6 and the bottom surface of the wafer 5 is not uncontrollably evacuated by a pump but, rather, a particular pressure that does not produce large contact thermal resistance, causing inconveniences, is retained controllably. Such a pressure is different, depending on the exposure conditions. For example, if the proximity gap of the exposure conditions in Table 1 is 50 microns, then R ≦ 1.01 [deg·cm 2 /W] and, thus, the pressure should be not less than 26664 Pa (200 Torr). It is to be noted here that the pressure difference between both sides of the wafer 5 has to be sufficient for the holding of the wafer 5 by the wafer chuck 6 and, to this end, the pressure to be applied to the front surface of the wafer is controlled as required, simultaneously with the control of the pressure in the reduced pressure duct 11 of the wafer chuck 6. Approximately, a pressure difference not less than 13332 - 19998 Pa (100 - 150 Torr) is necessary. Referring back to Figure 1, there is shown an X-ray exposure apparatus to which an embodiment of the present invention is applied. In Figure 1, denoted at 1 are X-rays as produced by an accumulating ring or the like, not shown; at 3 is a chamber the inside of which is occupied by a gas such as He, for example, transmissive to the X-rays 1, wherein the X-rays 1 pass through a beryllium (Be) window 2 and enter into the chamber 3; at 4 is a mask; at 5 is a wafer onto which a pattern formed on the mask 4 is to be printed through the exposure to the X-rays 1; at 6 is a wafer chuck for holding the wafer 5 by attraction; at 7 is a passageway which is formed in the wafer chuck 6 and through which constant temperature water circulates to maintain the temperature of the wafer chuck 6 constant. The passageway 7 is communicated with an outlet port 9 from which the constant temperature wafer flows into a constant temperature vessel 10 and, after being adjusted to a predetermined temperature, the water flows again into the passageway 7 from an inlet port 8. By this circulation, the wafer chuck 6 can be maintained at a constant temperature. Further, the wafer chuck 6 is provided with a reduced pressure duct 11 for the holding of the wafer 5. The reduced pressure duct 11 is communicated with a pressure gauge 13a through a drawing duct 12 and, also, the reduced pressure duct is communicated with a vacuum supplying pump 17 and an He gas supplying bomb 16a through gas adjusting valves 15a and 15b, respectively. The quantity of gas supply to the reduced pressure duct 11 can be determined by the degree of opening/closing of each of the adjusting valves 15a and 15b. A signal corresponding to a pressure value measured by the pressure gauge 13a is applied to a central processing unit (CPU) 20. On the basis of the signal from the pressure gauge 13a, the CPU 20 supplies a signal to a controller 14a, in response to which the controller 14a controls the opening/closing of the gas adjusting valves 15a and 15b, by which the pressure in the reduced pressure duct 11 can be maintained at a predetermined value (for example, 6666 Pa (50 Torr)). Namely, the contact thermal resistance between the wafer 5 and the wafer chuck 6 is maintained at a desired value. Similarly, the He gas pressure in the chamber 3 is maintained constant by controlling the opening/closing of the gas adjusting valve 15c through the controller 14b in response to a signal from another pressure gauge 13b, measuring the pressure within the chamber 3, applied to the CPU 20, namely, by controlling the quantity of the He gas supplied to the chamber 3 from the bomb 16b. In this embodiment, first the CPU 20 calculates and determines the pressure to be established in the reduced pressure duct 11 on the basis of various data, such as those shown in Table 1 and Figure 3, as inputted into a memory of the CPU 20 beforehand and of the three equations mentioned above. It is to be noted that those data based on the specification of the exposure apparatus and the state of exposure, necessary for these three equations for calculation of the contact thermal resistance R, are inputted into the CPU 20 beforehand. The pressure in the reduced pressure duct 11 is different, depending on the pressure difference between both sides of the wafer 5 and the material of the wafer chuck 6. For example, if the pressure difference is 19998 Pa (150 Torr) and the wafer chuck 6 is made of a SUS material, from Figure 3 the pressure to be provided in the reduced pressure duct 11 is determined to be equal to about 50 Torr, and the gas adjusting valves 15a and 15b are opened/closed so as to maintain the determined pressure. The He gas pressure in the chamber 3 is determined to be equal to 26664 Pa (200 Torr) which corresponds to the sum of the pressure of 6666 Pa (50 Torr) in the reduced pressure duct 11 and the pressure difference of 19998 Pa (150 Torr) for the holding of the wafer 5. Similarly to the pressure control of the reduced pressure duct 11, the He gas pressure is controlled to be maintained at the determined pressure, by means of the gas adjusting valve 15c. As regards the contact thermal resistance R, there are cases where the resistance changes not only with the pressure Pn of the gas in the reduced pressure duct 11 but also with the flatness of the wafer 5 and/or the material of the wafer chuck 6. Further, since the contact thermal resistance R to be set changes with the strength of the X-rays 1 or the like, also in this respect, it is necessary to adjust the pressure Pn of the reduced pressure duct 11. Where the pressure Pn of the reduced pressure duct 11 is changed in accordance with the conditions and if the pressure difference Pw for the holding of the wafer 5 by the wafer chuck 6 is to be unchanged, the ambience pressure Pc (= Pn + Pw) in the chamber 3 changes with the change in the pressure Pn. However, if the pressure Pc in the chamber 3 changes, the transmission factor to the X-rays 1 passing therethrough changes. Thus, there is a possibility that a desired exposure amount is not obtained at the time of exposure of the wafer 5 to the mask 4. In order to avoid this, the following two methods may be adopted. (1) The first is that: the exposure time of the mask 4 and the wafer 5 to the X-rays 1 is changed in accordance with the X-ray transmission factor or the pressure Pc in the chamber 3 as measured by the pressure gauge 13b. The exposure time can be changed by means of a shutter (not shown), for example, provided at an upstream side of the mask 4. (2) The second is that: when a maximum of the pressure Pn to be established in the reduced pressure duct 11 in accordance with various exposure conditions is denoted by P (n-Mx) , the pressure Pc in the chamber 3 is set so as to satisfy the following relation: Pc = Pw + P (n-Mx) or Pc > Pw + P (h-Mx) If this is done, the pressure Pc in the chamber 3 can be maintained constant even if the pressure Pn of the reduced pressure duct 11 changes with the exposure conditions. As a result, a desired exposure amount is always obtainable. It will be understood from the foregoing that an important feature of the present invention resides in that a gas which serves as a heat conducting medium is present between the wafer 5 and the wafer chuck 6, at a predetermined pressure. Thus, the He bomb 16a in Figure 1 is not always necessary. The structure may be modified such as shown in Figure 4, for example, wherein the drawing duct 12 is coupled to a switching valve 18, a gas adjusting valve 15a and a pump 17 in this order, such that by increasing the restriction conductance of the gas adjusting valve 15a, the pressure in the reduced pressure duct 11 is maintained at a desired value. In this example, by switching the valve 18, the reduced pressure duct 11 can be communicated with the chamber 3 whereby the reduced pressure duct 11 and the chamber 3 can be maintained at the same pressure. Thus, the wafer 5 can be easily unloaded from the wafer chuck 6. Also, while in the above-described embodiment the pressure gauge 13a for measurement of the pressure (vacuum level) of the reduced pressure duct 11 is coupled to the conduit 12 for the gas supplying system or the gas discharging system, the piping for the pressure gauge 13a may be separated from the conduit 12, such that the pressure in the reduced pressure duct 11 may be measured without intervention of the conduit 12. In that occasion, the pressure gauge 13a can measure the pressure of the reduced pressure duct 11 without being affected by the conductance of the conduit 12, for example, and therefore, more accurate measurement is possible. Further, it is not necessary that the pressure in the chamber 3 is maintained at a reduced pressure of 26664 Pa (200 Torr) as mentioned hereinbefore. It is sufficient that the pressure difference between both sides of the wafer 5 is enough to hold the wafer 5. Thus, the pressure may be equal to or higher than the atmospheric pressure. While, in the foregoing, description has been made of an example wherein the invention is applied to an exposure apparatus, the applicability of the invention is not limited to such field. Rather, the invention is applicable widely to various apparatuses wherein a sample is to be maintained at a constant temperature and wherein the sample is to be held by using an attracting force resulting from a pressure difference produced between both sides of the sample. In accordance with the present invention, as described hereinbefore, the pressure in the space between a bottom surface of a substrate and a substrate holding table is not uncontrollably evacuated but, rather, a particular pressure that does not produce large contact thermal resistance, causing various inconveniences, is positively and controllably retained. As a result, it is possible to suppress temperature rise of the substrate. While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the scope of the following claims.

Claims: A method of holding a substrate (5) having first and second major surfaces for exposure of said second major surface with radiation, wherein the first major surface is attracted to a holding chuck (6) by a pressure difference produced between the first major surface and the second major surface, characterised in that a pressure applied by a supply of gas to a clearance between the first major surface and the holding chuck is measured, wherein during the exposure of the second major surface of the substrate, the pressure difference between the first major surface and the second major surface of the substrate and the pressure applied by the supply of gas are controlled so that the pressure difference is maintained at a magnitude sufficient to keep the substrate attracted to the holding chuck and so that the pressure applied by the supply of gas is at a magnitude sufficient to maintain a contact thermal resistance between the substrate and the holding chuck at a predetermined value. A method as claimed in claim 1, characterised in that the supply of the gas and the pressure difference are each controlled by a valve. A method as claimed in claim 1 or 2, characterised in that the temperature of the holding chuck is controlled. A method as claimed in claim 1, 2 or 3, characterised in that the holding chuck (6) is disposed in a chamber (3). A method as claimed in claim 4, characterised in that the pressure within the chamber is controlled. A method according to any one of claims 1-5, characterised in that the substrate is held at a position adjacent a mask having a pattern and the pattern is transferred to the substrate by the exposure. A method according to any one of claims 1-6, characterised in that the substrate comprises a semiconductor wafer. A method as claimed in any one of claims 1-7, characterised in that the gas comprises helium. A method according to any one of claims 1-8, characterised in that the radiation comprises X-rays. A method according to any one of claims 1-9, characterised in that the holding chuck holds the substrate vertically. A method of manufacturing a semi-conductor device comprising the steps of holding a substrate (5) using the method of any preceding claim, and exposing said substrate (5) to radiation at a predetermined exposure position on said second major surface. A substrate holding device comprising: a holding chuck (6) having a reduced pressure passageway (12); a first pressure gauge (13a) for measuring the pressure in said reduced pressure passageway (12); pump means (17) for reducing the pressure in said reduced pressure passageway (12) to produce a pressure difference between a first surface of a substrate (5) to be held by said holding chuck (6) and a second surface of the substrate (5) to be exposed to radiation; valve means (15a) which can be opened or closed to control the pressure in said reduced pressure passageway (12); characterised by a chamber (3) containing said holding chuck (6) and said substrate (5); gas supply means (16b) for supplying gas to said chamber (3) to achieve a predetermined pressure; a second pressure gauge (13b) for measuring the pressure in said chamber (3); and pressure control means (14a,14b,20) adapted to receive pressure measurements from said first and second pressure gauges (13a,13b) and, during the exposure of said second surface of said substrate (5) to control said valve means (15a) and said gas supply means (16b) to maintain the pressure difference between said first and second surfaces of said substrate (5) at a magnitude sufficient to keep said substrate (5) attracted to said holding chuck (6), and to control the pressure in said reduced pressure passageway (12) to maintain a contact thermal resistance between said substrate (5) and said holding chuck (6) at a predetermined value. A device as claimed in claim 12 including a second gas supply means (16a) for supplying gas to said reduced pressure passageway (12) and second valve means (15b) for controlling the supply of gas from said second gas supply means (16a) to said reduced pressure passageway (12), said pressure control means (14a,14b,20) being adapted to control said valve means (15a) and said second valve means (15b) to control the pressure in said reduced pressure passageway (12). A device as claimed in claim 12 including switching valve means (18) for switchably connecting said reduced pressure passageway (12) to said valve means (15a) and said pump means (17) or to said chamber (3), said pressure control means (14a,14b,20) being adapted to control said switching valve means (18) and said valve means (15a) to control the pressure in said reduced pressure passageway (12). Exposure apparatus for manufacturing semi-conductor devices comprising the substrate holding device of any one of claims 12 to 14, a mask (4) containing a semi-conductor pattern, and a radiation transparent window (2) provided in said chamber (3) to allow exposure radiation to enter said chamber (3) to expose said second surface of said substrate (5) through said mask (4).

Inventors: AMEMIYA MITSUAKI, HARA SHINICHI, SAKAMOTO EIJI, AMEMIYA, MITSUAKI, HARA, SHINICHI, SAKAMOTO, EIJI
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1990
Main classification: H01L21
IPC codes: H01L21, G03F7