A wafer supporting apparatus

Abstract: A wafer chuck (2) usable with a semiconductor exposure apparatus (Fig.2A) wherein a mask and a semiconductor wafer (1) are placed in a vacuum ambience or a pressure-reduced gas ambience, and wherein the wafer (1) is exposed through the mask to radiation energy such as X-rays contained in a synchrotron radiation beam, by which the pattern of the mask is transferred onto the wafer (1). The wafer (1) is first attracted on the wafer supporting surface of the chuck (2) by vacuum attraction (21), and thereafter, the wafer (1) is attracted by electrostatic attraction force. Thereafter, the vacuum attraction is broken by supplying (24) a gas. Preferably Helium or other thermally conductive gas is admitted at this point. When the pattern of the mask is transferred onto the wafer (1), the wafer (1) is retained on the wafer supporting surface (20) by the electrostatic attraction force only. By this, the wafer supporting apparatus can correctly contact the wafer supporting surface (20) to the wafer (1) without being influenced by the undulation of the wafer (1). In addition, the heat produced in the wafer (1) during exposure can be removed efficiently by temperature controlled (23) water (11) supplied to the wafer supporting apparatus (2).

Description: The present invention relates to a wafer supporting apparatus, more particularly to the wafer supporting apparatus usable with a semiconductor exposure apparatus wherein a mask and a semiconductor wafer is disposed in a vacuum or a predetermined reduced ambience; a predetermined exposure energy, for example, X-rays contained in a radiation from SOR (Synthetic Orbit Radiation) is applied to the wafer through the mask, by which the pattern of the mask is transferred onto the wafer. The invention further relates to a method of supporting a wafer for an exposure process. A wafer supporting apparatus including a vacuum attracting means utilizing vacuum attracting force and a wafer supporting apparatus having an electrostatic attracting means utilizing electrostatic force, are known. Referring first to Figures 3A and 3B, there is shown a vacuum attracting type wafer supporting apparatus. Figure 3A is a top plan view thereof, and Figure 3B is a sectional view. On the top surface of the supporting apparatus 102, vacuum attracting grooves 104 are formed, the grooves communicating with unshown vacuum source through a vacuum pipe 103. The wafer 101 is placed on the top surface of the apparatus 102, that is, on the contact surface, and is vacuum-attracted by the vacuum grooves 104, wherein the ambient pressure to the top surface of the wafer 101 is effective to urge the wafer 101 to the apparatus 102. Referring then to Figures 4A and 4B, Figure 4A is a top plan view, and Figure 4B is a sectional view. On the top surface of the apparatus 202 an electrode 208 is disposed, and the top surface thereof is covered with an insulating layer 203. The wafer 201 is placed on the top surface (contact surface) of the insulating layer 203, and a voltage is applied to the electrode 208. By the application of the voltage, an electrostatic force is produced to attract the wafer on the contact surface. In the conventional vacuum attraction type wafer supporting apparatus, a vacuum layer exists between the wafer and the contact surface of the apparatus with the result of significantly large resistance to the contact heat transfer. For example, when the comparison is made between the vacuum attraction and the electrostatic attraction in the ambient condition of normal temperature and normal pressure, the resistance to the contact heat transfer between the wafer and wafer carrying surface is 10 -2 K.m 2 /W in the vacuum attraction, whereas it is 10 -4 K.m 2 /W in the electrostatic attraction, and therefore, the resistance is larger by approximately two orders in the vacuum attraction than in the electrostatic attraction. The heat produced in the wafer by absorption of exposure energy during the pattern transfer operation from the mask to the wafer is not easily released to the supporting apparatus in the vacuum attraction. When the apparatus is used with an exposure apparatus wherein the wafer is disposed in an ambience having a low thermal conductivity, such as vacuum ambience, the temperature rise of the wafer is a problem. Particularly when the exposure operation is performed with the mask and the wafer being placed closely to each other, the temperature rise of the mask as well as of the wafer is an additional problem. If the temperature of the mask rises, the pattern formed thereon is thermally deformed. In a wafer supporting apparatus of the electrostatic attraction type, when the electrode carrying the wafer on the attracting surface is supplied with the voltage, the attraction force is larger in the portion where the gap between the wafer and the attraction surface is smaller than where it is larger. Since the attraction force is smaller where the gap is large, the attraction force over the surface of the wafer is not uniform due to undulation. In some cases, the undulation is not corrected. In an extreme case, the wafer is demounted from the supporting apparatus. Reference is made to IBM Technical Disclosure Bulletin, vol. 27, no. 10B, March 1985, page 6222 which discloses a combined vacuum and electrostatic wafer chuck capable of electrostatically attracting non-conductive work pieces and/or workpieces which are not electrically contacted. An object of the present invention is to provide a wafer supporting apparatus wherein the contact heat transfer resistance to the wafer is small, and wherein the wafer is positively and assuredly supported on the supporting surface. According to the invention there is provided wafer supporting apparatus, comprising a support structure having a supporting surface for supporting a wafer; vacuum attraction means for applying vacuum from a vacuum source to the rear side of the wafer to attract said wafer to said supporting surface; and electrostatic attraction means for attracting the wafer onto said supporting surface by electrostatic force; characterised by a pressure sensor connected to said vacuum attraction means, a valve for controlling the application of said vacuum source, and a control means adapted to operate in sequence said valve to apply vacuum to said wafer, to switch on said electrostatic attraction means, and to operate said valve to release said vacuum and to permit communication of the rear side of the wafer with ambient gas on receipt by said control means of a signal from said pressure sensor indicating that the pressure at the rear side of the wafer has reached a predetermined level. Further according to the invention there is provided a method of supporting a wafer for the purpose of an exposure process, comprising a) supporting the wafer on a support structure having vacuum attraction means and electrostatic attraction means, said vacuum attraction means and said electrostatic attraction means being controllable by control means and a pressure sensor, b) applying a vacuum to the rear side of the wafer by actuation of said vacuum attraction means by said control means for attracting the wafer onto the wafer supporting surface of said support structure until a predetermined pressure level is detected by said pressure sensor, c) actuating said electrostatic attraction means by said control means, d) thereafter de-activating said vacuum attraction means by said control means to release the vacuum, e) exposing the rear side of the wafer to ambient gas, and f) then applying exposure energy to the wafer while the latter is secured only by said electrostatic attraction means. In operation, the wafer is attracted by both of the vacuum attracting force and the electrostatic attracting force on the supporting surface. Thereafter, the vacuum attracting means is supplied with an ambient gas while the wafer continues to be attracted by the electrostatic attracting means, whereby the contact heat transfer resistance is reduced. These and other objects, 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 the accompanying drawings: Figure 1 is a top plan view of a wafer supporting apparatus according to an embodiment of the present invention. Figure 2A is a sectional view taken along a line A-A′ of Figure 1. Figure 2B is a sectional view taken along a line B-B′ of Figure 1. Figure 3A is a top plan view of a conventional vacuum attraction type wafer supporting means. Figure 3B is a sectional view of the apparatus of Figure 3. Figure 4A is a top plan view of a wafer supporting apparatus of a conventional electrostatic attraction wafer supporting apparatus. Figure 4B is a sectional view of an apparatus of Figure 1. The preferred embodiment will be described in conjunction with the accompanying drawings. In the Figure, the wafer supporting apparatus is shown as being used with a semiconductor exposure apparatus, although not shown, wherein a mask and a wafer are disposed in a vacuum ambience or a pressure-reduced ambience, and wherein a predetermined exposure energy such as X-rays contained in a synchrotron radiation beam is projected through the mask to the wafer, by which the pattern formed in the mask is transferred onto the wafer. In Figures 1, 2A and 2B, a reference numeral 1 designated a semiconductor wafer onto which the pattern of the mask is to be transferred or printed. The wafer supporting apparatus of this embodiment includes a wafer supporting main assembly 2 for supporting the wafer 1 at an exposure position where the mask pattern is transferred thereonto and an insulating layer 3 constituting a contact surface with the wafer 1. The main assembly 2 is made of ceramic material such as Al 2 O 3 or SiC or metal such as SUS, Al. The insulating layer 3 is made of a material having high dielectric constant such as Al 2 O 3 . The thickness thereof is approximately 50 - 400 microns (micrometers). If the thickness of the insulating layer 3 is smaller, the electrostatic attraction force is larger. However, when the high voltage is applied, the dielectric breakdown can easily occur. On the other hand, if it is large, the electrostatic force is small. In this embodiment, fused Al 2 O 3 is applied, and then it is abraded into the thickness of 100 microns. The top surface of the insulating layer 3 is formed into a contact surface 20 for contact with the wafer 1. The contact surface 20 is provided with vacuum attraction grooves 4 as shown in Figure 1 in the form of concentric circles. The vacuum attraction grooves 4 communicate with vacuum source 21 through a vacuum pipe 14. The vacuum pipe 14 is provided adjacent to the main assembly 2 with a pressure sensor 6 for detecting the pressure in the pipe 14. Between the pressure sensor 6 and the vacuum source 21, there is disposed a three way valve 7, by which the ambient gas or gases around the apparatus 24 can be introduced through a gas inlet 24 into the pipe at the pressure sensor 6 side. Below the insulating layer 3 there is an electrode 8. The electrode 8 is electrically connected to a high voltage source 10 through a switch 9. By closing the switch 9, a high voltage is applied to the electrode 8, by which an electrostatic force is produced between the apparatus and the wafer 1. Thus, the wafer is attracted and fixed on the insulating layer 3. When the material of the main assembly 2 is electrically conductive, an additional insulating layer provided on the bottom surface of the electrode 8. In the main assembly 2, a water circulation region 11 is formed for circulating temperature control liquid (water) (temperature control medium). Into the circulation region 11, the cooling water controlled in its temperature at a predetermined level is supplied from a temperature controlled water source 23. The cooling water flows through the circulation region 11 to absorb the heat from the main assembly 2 and the wafer 1, and thereafter, is discharged through an outlet 13. By this, the temperature rise during the exposure operation can be suppressed to maintain the temperature of the wafer 1 and the main assembly 2 at a predetermined level. The temperature-controlled water source 23 is controlled in its temperature in response to instructions from a controller 22, and also is controlled in the flow rate. In operation, the wafer is attracted in the following steps: (1) The wafer 1 is placed on the insulating layer 3 of the main assembly 2, thereafter, the controller 22 switches the three way valve 7 to connect the pipe 14 with the vacuum source 21, and starts operation of the vacuum supply source 21. (2) By this, the wafer 1 is vacuum-attracted by the vacuum attracting grooves 4 between the wafer 1 and the main assembly 2, so that the wafer 1 is vacuum-attracted on the insulating layer 3. The controller 22 monitors through the pressure sensor 6 the pressure in the pipe 14, and confirms the attraction of the wafer 1 by the pressure in the pipe 14 beyond a predetermined level. (3) After the vacuum attraction of the wafer 2 is confirmed, the controller 22 closes the switch 9 to apply the high voltage to the electrode 8, by which the wafer 1 is securedly attracted to and retained on the main assembly 2 by both of the vacuum-attracting force and the electrostatic attracting force. (4) Then, the controller 22 stops operation of the vacuum source 21. Thereafter, the three way valve 7 is switched to open the gas introduction inlet 24 side to introduce the ambient gas or gases into the vacuum pipe 14, thus breaking the vacuum. By this, the ambient gas is introduced into between the wafer 1 and the contact surface of the insulating layer 3 of the main assembly 2, by which the resistance to the contact heat transfer therebetween reduces. The ambient gas is preferably He gas having a high thermal conductivity. Thereafter, the exposure operation is started, and during the exposure wafer 1 is kept retained only by the electrostatic attraction force. In this embodiment, the ambient gas around the main assembly 2 is introduced into the vacuum pipe 14 after the wafer is electrostatically attracted. However, it is a possible alternative that a gas supplying source is connected the gas inlet 24 to introduce another gas or gases into the pipe 14. When, the ambient gas has relatively low thermal conductivity, such as air or nitrogen, it is possible that a highly thermally conductive gas such as He can be introduced into the vacuum pipe 14 from the gas inlet 24, by which the resistance to the contact heat transfer between the wafer 1 and the main assembly 2 of the supporting apparatus can be reduced. As described in the foregoing, according to the present invention, the wafer supporting means is provided with vacuum attraction means and electrostatic attraction means, by which the wafer can be assuredly retained and fixed on the wafer supporting surface. The undulation of the wafer can be corrected along the attracting surface of the supporting apparatus. In addition, the temperature rise of the wafer during the exposure operation of the wafer to the mask pattern can be suppressed. Therefore, the heat of the mask and the wafer can be removed efficiently, thus minimizing the thermal deformation of the mask pattern. 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 scope of the following claims.

Claims: Wafer supporting apparatus, comprising a support structure having a supporting surface for supporting a wafer; vacuum attraction means (4, 7, 14, 21) for applying vacuum from a vacuum source to the rear side of the wafer to attract said wafer to said supporting surface; and electrostatic attraction means (8, 9, 10) for attracting the wafer onto said supporting surface by electrostatic force; characterised by a pressure sensor (6) connected to said vacuum attraction means, a valve (7) for controlling the application of said vacuum source, and a control means (22) adapted to operate in sequence said valve to apply vacuum to said wafer, to switch on said electrostatic attraction means, and to operate said valve (7) to release said vacuum and to permit communication of the rear side of the wafer with ambient gas on receipt by said control means of a signal from said pressure sensor (6) indicating that the pressure at the rear side of the wafer has reached a predetermined level. An apparatus according to claim 1, characterised in that said vacuum attraction means includes grooves (4) formed on said supporting surface, a vacuum source (21) for applying vacuum to said grooves and a gas inlet (14) for supplying a gas into said grooves (4). An apparatus as claimed in claim 1 or 2, characterised by temperature control means (11, 23) for circulating a temperature-controlled medium in said support structure. An apparatus as claimed in claim 3, characterised in that said control means (22) also controls said temperature control means (11, 23). An exposure apparatus including means for applying exposure energy onto and through a mask, by which a pattern formed in the mask is transferred onto the wafer, including a wafer supporting apparatus as claimed in any one of claims 1 to 4. A method of supporting a wafer for the purpose of an exposure process, comprising a) supporting the wafer on a support structure having vacuum attraction means and electrostatic attraction means, said vacuum attraction means and said electrostatic attraction means being controllable by control means (22) and a pressure sensor (6), b) applying a vacuum to the rear side of the wafer by actuation of said vacuum attraction means by said control means for attracting the wafer onto the wafer supporting surface of said support structure until a predetermined pressure level is detected by said pressure sensor, c) actuating said electrostatic attraction means by said control means, d) thereafter de-activating said vacuum attraction means by said control means to release the vacuum, e) exposing the rear side of the wafer to ambient gas, and f) then applying exposure energy to the wafer while the latter is secured only by said electrostatic attraction means. A method as claimed in claim 6, characterised in that said control means (22) sequentially controls a valve (7) to apply said vacuum to grooves (4) formed in the supporting surface of said support structure and to introduce the gas through a gas inlet (24) to said grooves after said electrostatic attraction means is actuated. A method as claimed in claim 6 or 7, characterised in that the temperature of said support structure is controlled by a temperature controlled medium passing through the support structure.

Inventors: HARA SHINICHI, SAKAMOTO EIJI, SHIMODA ISAMU, UZAWA SHUNICHI, HARA, SHINICHI, SAKAMOTO, EIJI, SHIMODA, ISAMU, UZAWA, SHUNICHI
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1989
Main classification: H01L21
IPC codes: H01L21, G03F7