Abstract:
Wafer chucks and related substrate-holding devices are disclosed for use in holding the substrate while any of various processes are being performed on the substrate. For example, the devices are useful for holding a semiconductor wafer during microlithography performed on the wafer, especially in a vacuum environment. The wafer chucks can include devices for confirming that the substrate is adhered completely and properly to the “adhesion surface” of the wafer chuck before commencing flow of a heat-transfer gas to the wafer chuck. Such status-confirming devices can be, e.g., height gauges or electrical contacts that measure an electrical property that changes with changes in contact pressure of the contacts against the substrate. The wafer chucks can include devices that compensate for faulty adhesion of the substrate to the wafer chuck, such as devices that change pressures in ducts that supply and remove heat-transfer gas from a channel(s) located in the adhesion surface of the wafer chuck, so as to compensate for increased leak rates of heat-transfer gas into the environment surrounding the wafer chuck.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention pertains to microlithography (transfer of a pattern, defined on a reticle or mask, onto a sensitive substrate). Microlithography is a key technology used in the fabrication of semiconductor integrated circuits, displays, micromachines, and the like. More specifically, the invention pertains to devices (termed “wafer chucks”), to which the substrate (“wafer”) is mounted, that hold and move the substrate during microlithographic exposure. Even more specifically, the invention pertains to wafer chucks and related substrate-holding devices operable to correct instances of insufficient adhesion of the substrate to the substrate-holding device.  
         BACKGROUND OF THE INVENTION  
         [0002]    During microlithographic exposure of a sensitive substrate (“wafer”) the wafer typically is mounted to and held by a “wafer chuck.” Microlithography performed using a charged particle beam must be performed in a subatmospheric pressure (“vacuum”) environment; hence, the wafer chuck must be capable of holding the wafer in such an environment. Most conventional wafer chucks intended for use in a vacuum environment are configured to hold the wafer using electrostatic force. The surface of the wafer chuck to which the wafer (i.e., the downstream-facing surface of the wafer) is mounted is termed the “adhesion surface” of the wafer chuck.  
           [0003]    During exposure of a wafer using a charged particle beam, the exposure beam is incident with high energy on the “sensitive” surface (upstream-facing resist-coated surface) of the wafer. Consequently, the wafer tends to experience heating, which can cause undesired thermal expansion of the wafer. Thermal expansion of the wafer can degrade the accuracy with which a pattern is transferred to the sensitive surface. Under extreme circumstances of wafer heating, the wafer can detach from or shift position on the adhesion surface.  
           [0004]    A conventional method of reducing wafer heating is to configure the adhesion surface with grooves or channels that open onto the adhesion surface and the downstream-facing surface of the wafer. A heat-transfer gas such as helium is conducted through the channels, whenever the wafer is mounted to the adhesion surface, to dissipate heat from the wafer and thus reduce thermal expansion of the wafer.  
           [0005]    To ensure that the wafer remains attached to the adhesion surface as the heat-transfer gas is passed through the channels, the pressure of the heat-transfer gas passing through the channels is regulated. In other words, the pressure of the gas must be less than a pressure, opposing the electrostatic force, sufficient to detach the wafer from the adhesion surface. Meanwhile, parameters that determine the quantity of heat transferred from the wafer to the wafer chuck by the heat-transfer gas include the thermal conductance of the gas, the gas pressure, and the length of the channel(s) through which the gas passes. For example, if the gas pressure is sufficiently low that the mean free path of the gas molecules is longer than a transverse dimension of the channel, then the thermal conductivity of the heat-transfer gas increases nearly proportionally to the gas pressure. On the other hand, if the mean free path is shorter than a transverse dimension of the channel, then the thermal conductivity is not proportional to the gas pressure.  
           [0006]    Because the wafer chuck normally is located in a subatmospheric pressure environment, as the pressure of the heat-transfer gas in the channel increases, adhesion of the wafer to the adhesion surface of the wafer chuck weakens. In the worst case, the wafer actually detaches from the wafer chuck. Hence, it is important to maintain the pressure of the heat-transfer gas in the channel below a threshold that otherwise would result in detachment of the wafer from the adhesion surface.  
           [0007]    The mean free path of molecules of the heat-transfer gas is obtained from an estimate of the pressure of the heat-transfer gas. In view of this, it is desirable to configure the channel (in the adhesion surface and located between the wafer chuck and the downstream-facing surface of the wafer) to have transverse dimensions that are equal or nearly equal to the mean free path.  
           [0008]    With a conventional electrostatic wafer chuck, after the chuck is charged electrostatically, the wafer is assumed to be adequately adhered to the adhesion surface and the flow of heat-transfer gas through the channel begins. But, if the wafer in fact is not adhered adequately to the wafer chuck, even if the pressure of the heat-transfer gas is regulated “normally,” a substantial risk exists that the wafer will “float” and laterally shift position on the adhesion surface. Other adverse consequences are also possible, such as the wafer actually falling off the wafer chuck. If any of these adverse events occurs, then the vacuum inside the chamber enclosing the wafer chuck must be broken and the wafer removed by hand. Afterward, the process of re-establishing the vacuum in the chamber and re-mounting the wafer to the wafer chuck must be performed, which results in lengthy equipment down-time.  
           [0009]    Other possible adverse conditions are the presence of particulate debris between the downstream-facing surface of the wafer and the adhesion surface as the wafer is resting on the adhesion surface, and poor planarity or flatness of the wafer itself. As noted above, the flow of heat-transfer gas into the channel is regulated to maintain a particular target pressure of the gas in the channel under normal conditions. But, either of the adverse conditions noted above essentially opens the channel and allows excess leakage of heat-transfer gas from the channel into the vacuum chamber.  
         SUMMARY OF THE INVENTION  
         [0010]    In view of the disadvantages of conventional wafer chucks as summarized above, an object of the invention is to provide substrate-holding devices (generally termed herein “wafer chucks”) configured to prevent insufficient adhesion of the wafer to the wafer chuck. Another object is to provide microlithography apparatus including such improved wafer chucks.  
           [0011]    To such ends and according to a first aspect of the invention, substrate-holding devices are provided. An embodiment of such a device includes a wafer-chuck body that defines an adhesion surface and comprises an electrostatic electrode. The adhesion surface is configured to contact a downstream-facing surface of a substrate whenever the substrate is being held by the substrate-holding device by an electrostatic force generated by the electrode. The adhesion surface defines a channel that is configured, whenever the substrate is adhered to the adhesion surface by the electrostatic force, to provide a conduit for a heat-transfer gas. Hence, whenever the heat-transfer gas is flowing through the conduit, the gas contacts and removes heat from the downstream-facing surface of the substrate. The device also comprises a gas-supply system and a substrate-adhesion-confirmation device. The gas-supply system is connected to the channel and configured to supply a flow of the heat-transfer gas to the channel. The substrate-adhesion-confirmation device is situated and configured to detect whether the substrate is adhered to the adhesion surface. The device also includes a controller connected to the substrate-adhesion-confirmation device and to the gas-supply system. The controller is configured to cause the gas-supply system to supply the flow of the heat-transfer gas to the channel after the substrate-adhesion-confirmation device has confirmed adhesion of the substrate to the adhesion surface.  
           [0012]    By way of example, the substrate-adhesion-confirmation device can comprise a height gauge situated and configured to measure an elevation of the substrate. Alternatively, the substrate-adhesion-confirmation device can comprise multiple grounding pins each situated and configured to contact the substrate electrically in a manner whereby a contact resistance of the electrical contact varies with contact pressure exerted by the respective grounding pin on the substrate. In this latter configuration, a power supply is provided that is connected via an electrical circuit to the grounding pins. The device, utilizing the power supply, can be configured to provide the requisite confirmation by any of the following schemes: (1) grounding of the substrate via the grounding pins and measuring changes in contact resistance of the pins with changes in contact pressure of the substrate against the grounding pins; (2) impressing a voltage between any pair of grounding pins and measuring voltage changes with changes in contact pressure of the substrate against the grounding pins; and (3) flowing an electrical current between any two of the pins and measuring the current. In general, if the contact pressure of a pin against the downstream-facing surface changes, then the contact resistance between the pin and the substrate changes, leading to a corresponding change in, e.g., electrical current between any two pins. Also, since the substrate is grounded by the pins, charging of the substrate during irradiation by a charged particle beam can be prevented.  
           [0013]    If a grounding pin is not actually contacting the downstream-facing surface of the substrate, then electrical current will not flow through the circuit including the pin. Thus, it can be checked readily whether a pin is contacting the downstream-facing surface firmly.  
           [0014]    Hence, according to the invention, it is not simply presumed that the substrate is adhered properly to the holding device after the electrode is energized. Rather, a confirmation is made that the substrate is adhered properly to the holding device. If no confirmation is made, then heat-transfer gas is not delivered to the channel. Consequently, problems associated with poor wafer chucking are avoided.  
           [0015]    The device also can include a substrate-alignment device situated and configured to maintain a predetermined alignment position of the substrate relative to the adhesion surface under conditions in which the substrate is not actually adhered to the adhesion surface. The substrate-alignment device can comprise multiple alignment pins situated around the adhesion surface and configured to contact a respective edge of the substrate if the substrate moves laterally relative to the adhesion surface.  
           [0016]    Desirably, the alignment pins are made of a non-magnetic metal, and are attached to the wafer chuck around a perimeter of the adhesion surface. By making the alignment pins of a non-magnetic metal, a charged particle beam impinging on the substrate being held by the substrate-holding device is unaffected by extraneous magnetic fields that otherwise would be formed.  
           [0017]    According to another aspect of the invention, methods are provided for holding a substrate to allow performing a process on a process surface of the substrate. In an embodiment of such a method, a wafer chuck is provided that comprises a chuck body, an electrostatic electrode, and an adhesion surface on the chuck body. The adhesion surface defines a channel configured, whenever the substrate is adhered to the adhesion surface, to provide a conduit for a heat-transfer gas that, when flowing through the conduit, contacts and removes heat from the downstream-facing surface of the substrate. The substrate is placed on the adhesion surface, and the electrode is energized to cause the electrode to generate an electrostatic force intended to attract the substrate toward the adhesion surface. A confirmation is made that the substrate is adhered to the adhesion surface. After making the confirmation, a flow of the heat-transfer gas through the channel is commenced.  
           [0018]    In another method embodiment, a wafer chuck is provided that includes the components summarized above as well as a substrate-alignment device. The substrate-alignment device is situated relative to the chuck body and is configured to contact a respective edge of the substrate if the substrate moves laterally relative to the adhesion surface. The substrate is placed on the adhesion surface and the electrode is energized. A flow of the heat-transfer gas through the channel is commenced. Using the substrate-alignment device, respective edges of the substrate are contacted as required to prevent the substrate from laterally sliding off the adhesion surface as a result of the heat-transfer gas flowing through the channel.  
           [0019]    The foregoing methods can be utilized in conjunction with a method for manufacturing a microelectronic device on a substrate. For example, the manufacturing method can be a microlithographic method.  
           [0020]    Another embodiment of a substrate-holding device comprises a wafer chamber defining an interior space. A pump is connected to the wafer chamber and configured to evacuate the interior space to a predetermined vacuum level. A wafer-chuck is situated within the interior space. The wafer chuck defines an adhesion surface and comprises an electrostatic electrode. The adhesion surface is configured to contact a downstream-facing surface of a substrate whenever the substrate is being held by the substrate-holding device (by an electrostatic force generated by the electrode). The adhesion surface defines a channel configured, whenever the substrate is adhered to the adhesion surface by the electrostatic force, to provide a conduit for a heat-transfer gas that, when flowing through the conduit, contacts and removes heat from the downstream-facing surface of the substrate. The device includes a gas-inlet conduit connected to the channel and to a supply of the heat-transfer gas, wherein the gas-inlet conduit is configured to conduct the heat-transfer gas into the channel. The device includes a gas-evacuation conduit connected to the channel and to an evacuation pump, wherein the gas-evacuation conduit is configured to evacuate, as urged by the evacuation pump, the heat-transfer gas from the channel. The device includes a seal situated relative to the substrate, as held on the adhesion surface, and the channel.  
           [0021]    The seal is configured at least to limit an amount of heat-transfer gas flowing from the channel to the interior space whenever the substrate is on the adhesion surface. The device includes a first pressure detector situated and configured to measure a vacuum level in the interior space, and a second pressure detector situated and configured to measure a pressure within the gas-inlet conduit. The device also includes a controller connected to the first and second pressure detectors, to the supply of heat-transfer gas, and to the evacuation pump. The controller is configured to regulate a flow rate of the heat-transfer gas though the gas-inlet conduit in response to a pressure, detected by the second pressure detector, within the gas-inlet conduit, and to regulate a flow rate of gas through the gas-evacuation conduit in response to a vacuum level in the interior space as detected by the first pressure detector.  
           [0022]    The device summarized above also can include a third pressure detector connected to the gas-evacuation conduit and to the controller. With such a configuration, the controller is further configured to regulate the flow rate of heat-transfer gas through the gas-inlet conduit in response to a pressure, detected by the third pressure detector, in the gas-evacuation conduit.  
           [0023]    According to another aspect of the invention, charged-particle-beam (CPB) microlithography apparatus are provided that include a CPB-optical system situated and configured to direct a charged particle beam to form an image on a surface of a substrate. The apparatus also include a vacuum chamber defining an interior space, a vacuum pump connected to the wafer chamber and configured to evacuate the interior space to a predetermined vacuum level, and a wafer chuck. The wafer chuck is situated within the interior space. The wafer chuck defines an adhesion surface and includes an electrostatic electrode as summarized above. The wafer chuck also includes a gas-inlet conduit, a gas-evacuation conduit, a seal, a first pressure detector, a second pressure detector, and a controller, all as summarized above.  
           [0024]    In an embodiment, according to the invention, of a method for manufacturing a microelectronic device on a substrate, a method is included for holding the substrate to allow performing a process on a process surface of the substrate. In such a method, a vacuum environment is provided in which to hold the substrate while performing the process on the substrate. A predetermined vacuum level is established in the vacuum environment. In the vacuum environment a wafer chuck is provided that comprises a chuck body, an electrostatic electrode, an adhesion surface, and a seal as summarized above. The substrate is placed on the adhesion surface. The electrode is energized to cause the electrode to generate an electrostatic force intended to attract the substrate toward the adhesion surface. Heat-transfer gas is conducted at a first flow rate through a gas-inlet conduit to the channel. Heat-transfer gas is evacuated at a second flow rate from the channel through a gas-evacuation conduit. The vacuum level in the vacuum environment is detected to obtain respective data, and a gas pressure within the gas-inlet conduit is detected to obtain respective data. Responsive to this data, the first flow rate and the second flow rate are regulated.  
           [0025]    If adhesion of the substrate to the adhesion surface is poor, then the leakage of heat-transfer gas from the channel into the vacuum environment is increased compared to when adhesion is good and proper. Hence, whenever adhesion is poor, less vacuum is detected in the vacuum environment. The second flow rate is controlled (in this instance increased) to return to high vacuum in the vacuum environment, desirably bringing the vacuum level to a target value. Thus, leakage of heat-transfer gas from the channel is compensated for. Meanwhile, the first flow rate also is controlled to bring the detected pressure in the gas-inlet conduit at least closer to (desirably at) the target value.  
           [0026]    It is desirable to consider the pressure in the gas-evacuation conduit in the regulation of the flow rate in the gas-inlet conduit, so as to improve estimates of the pressure inside the channel.  
           [0027]    The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]    FIGS.  1 (A)- 1 (B) are elevational views depicting a wafer chuck according to the first representative embodiment, comprising height gauges for confirming the adhesion condition of the wafer to the adhesion surface of the wafer chuck. FIG. 1(A) is an exaggerated depiction of a “warped” wafer placed on the adhesion surface. FIG. 1(B) shows the wafer of FIG. 1(A) after being attracted electrostatically to the adhesion surface in a manner resulting in the wafer being attached firmly to the wafer chuck.  
         [0029]    [0029]FIG. 2 schematically depicts a wafer chamber (with wafer stage and wafer chuck) of a charged-particle-beam (CPB) microlithography apparatus comprising the wafer chuck of the first representative embodiment.  
         [0030]    [0030]FIG. 3 depicts the general structure of an electrostatic wafer chuck according to the second representative embodiment of the invention.  
         [0031]    FIGS.  4 (A)- 4 (C) depict enlarged views of the edges of the wafer chuck according to the third representative embodiment. FIG. 4(A) is a schematic elevational section showing a wafer adhered to the adhesion surface of the wafer chuck; FIG. 4(B) is a schematic elevational section showing a condition in which the pressure of the heat-transfer gas is excessive or the electrostatic force is insufficient, resulting in floating of the substrate relative to the wafer chuck; and FIG. 4(C) is a schematic plan view of the arrangement of alignment pins on the adhesion surface.  
         [0032]    [0032]FIG. 5 schematically depicts the overall structure of the wafer chamber and wafer chuck in the fourth representative embodiment of the invention.  
         [0033]    [0033]FIG. 6(A) is a plan view of a wafer chuck according to the fifth representative embodiment of the invention.  
         [0034]    [0034]FIG. 6(B) is an elevational section along the line A-A in FIG. 6(A).  
         [0035]    [0035]FIG. 7 is a flowchart of steps in a process for manufacturing a microelectronic device such as a semiconductor chip (e.g., integrated circuit or LSI), liquid-crystal panel, CCD, thin-film magnetic head, or micromachine, the process including performing microlithography using a microlithography apparatus according to the invention.  
     
    
     DETAILED DESCRIPTION  
       [0036]    The invention is described below in the context of representative embodiments, which are not to be regarded as limiting in any way. The embodiments are described in the context of using an electron beam as a representative charged particle beam. However, it will be understood that the general principles described herein are applicable with equal facility to use of another type of charged particle beam, such as an ion beam. Also, although normally not used in an optical microlithography apparatus, (i.e., a microlithography apparatus employing light as an energy beam), this invention can be incorporated and used with ready facility in an optical microlithography apparatus.  
       First Representative Embodiment  
       [0037]    This embodiment is shown in FIGS.  1 (A)- 1 (B) and  2 . More specifically, FIGS.  1 (A)- 1 (B) depict a wafer chuck operable to confirm the condition of adhesion of a wafer to the adhesion surface of the wafer chuck using a height gauge; and FIG. 2 schematically depicts structure associated with the wafer chamber of a charged-particle-beam (CPB) microlithography apparatus including the wafer chuck shown in FIGS.  1 (A)- 1 (B). In the following description, “top,” “bottom,” “left,” and “right” correspond to top, bottom, left, and right, respectively, in FIGS.  1 (A)- 1 (B) and  2 .  
         [0038]    Referring to FIG. 2, the CPB microlithography apparatus includes a wafer chamber  10 , which is a vacuum chamber. The wafer chamber  10  encloses a wafer stage  11  and a wafer chuck  12  mounted on the wafer stage  11 . The wafer stage  11  (with wafer chuck  12 ) is movable to and from an exposure position (left in the figure) and a wafer-exchange position (right in the figure). The wafer chamber  10  is connected to a vacuum-pump system  14  via a vacuum duct  13 . During operation of the CPB microlithography apparatus, the pressure in the wafer chamber  10  is reduced to and maintained at a specified vacuum level by the vacuum-pump system  14 . Extending from the wafer chamber  10  is a “lens column”  16  housing a CPB-optical system  16 A and CPB source  17  of the CPB microlithography apparatus. “Above” the wafer-exchange position, multiple height gauges  15  extend, relative to the wafer chamber, toward a wafer  18  on the wafer chuck  12 .  
         [0039]    The CPB source  17  is situated at the “top” of the lens column  16 . Whenever the wafer stage  11  is at the exposure position, the wafer  18  is situated “beneath” (downstream of) the lens column  16 . Thus, the “process surface”  18 A (upstream-facing surface typically coated with a layer of a suitable resist) of the wafer  18  can be irradiated by a charged particle beam CPB from the CPB source  17  and passing through the lens column  16 . Exposure of the wafer  18  in this manner forms a desired pattern on the process surface  18 A, as is well understood in the art.  
         [0040]    A wafer-exchange door  20  is provided in the right-hand wall of the wafer chamber  10 . The wafer-exchange door  20  thus defines a load-lock chamber  19  from which the wafer chamber can be isolated by the wafer-exchange door  20 . The load-lock chamber  19  contains a robotic transporter (not shown, but well understood in the art) that delivers new wafers and exchanges new wafers for processed wafers through the wafer-exchange door  20 . During wafer exchange, the wafer stage  11  is stopped at the wafer-exchange position near the wafer-exchange door  20 .  
         [0041]    The wafer stage  11  is movable, carrying the wafer chuck  12 , right and left in the wafer chamber  10 . The wafer chuck  12  comprises an adhesion surface  12 A (upstream-facing surface) to which the wafer  18  is held by electrostatic force generated by the wafer chuck  12 . To such end, electrodes  12 B are situated below the adhesion surface  12 A. The electrodes  12 B are connected via electrical wiring  21  to an adhesion-control system  22 . The adhesion-control system  22  causes the downstream-facing surface  18 B of the wafer  18  either to adhere to or be released from the adhesion surface  12 A by controlling the voltage supplied to the electrodes  12 B.  
         [0042]    Typically, a wafer  18  delivered into the wafer chamber  10  through the wafer-exchange door  20  is placed on the adhesion surface  12 A of the wafer chuck  12 . An electrostatic force is generated between the wafer  18  and the wafer chuck  12  by controlled application of electrical power to the electrodes  12 B by the adhesion-control system  22 . The electrostatic force causes the wafer  18  to be attracted to and to adhere to the adhesion surface  12 A.  
         [0043]    Channels  23  are defined (e.g., by machining) in the adhesion surface  12 A and extend “downward” into the mass of the wafer chuck  12 . The channels  23  are connected via a gas duct  24  and gas-flow regulator  26  to a supply source  25  of a heat-transfer gas (typically helium). During microlithographic exposure of the wafer  18 , heat-transfer gas is supplied through the gas duct  24  to the channels  23 . During the exposure, when the wafer normally would undergo heating due to impingement of the charged particle beam, heat is dissipated from the wafer to the heat-transfer gas and from the heat-transfer gas to the wafer chuck  12 . Thus, temperature increases of the wafer  18  are suppressed. This suppression of increases in wafer temperature controllably minimizes thermal expansion of the wafer  18  and maintains the accuracy by which the pattern is transferred to the process surface  18 A of the wafer  18 .  
         [0044]    The height gauges  15  (desirably two are provided) desirably are situated on an “upper” wall of the wafer chamber above the wafer-exchange position. The height gauges  15  are separated from each other by a specified distance (e.g., approximately the radius of the wafer  18 ), as shown in FIG. 2. The height gauges  15  measure the elevation of the wafer  18  whenever the wafer stage  11  is in the wafer-exchange position “below” the height gauges  15 . Specifically, one of the height gauges  15  measures the elevation of the center of the wafer  18  while the other of the height gauges  15  measures the elevation of the edge of the wafer  18 . The height gauges  15  can have any of various configurations and/or operating principles, such as using laser light, suitable for determining elevation of the wafer  18 .  
         [0045]    The process of confirming proper adhesion of the wafer  18  to the wafer chuck  12  is explained with reference to FIGS.  1 (A)- 1 (B). In the following discussion, numerical values are provided by way of example only and are not intended to be limiting in any way.  
         [0046]    In FIG. 1(A), the wafer  18  exhibits a “warp” (departure in the vertical direction from planarity) of, e.g., 100 μm. Whenever such a wafer  18  is measured by the height gauges  15 , a corresponding elevational difference will be detected at the center versus edge of the wafer  18 . After completing electrostatic adhesion of the wafer  18  to the adhesion surface  12 A of the wafer chuck  12 , as shown in FIG. 1(B), the measurement variation (“error”) from the center to the edge of the wafer  18  normally is reduced substantially, e.g., to about 0.1 μm. If the elevation of the adhered wafer  18  is measured by the height gauges  15 , the respective measured elevations at the center and edge of the wafer  18  are nearly equal to each other.  
         [0047]    In this example, if the difference in wafer elevation of the center versus the edge of the wafer  18  is greater than 0.3 μm, then a conclusion is reached that the wafer  18  is not adhered completely or properly to the adhesion surface  12 A of the wafer chuck  12 . Under such a condition, supply of the heat-transfer gas to the channels  23  is ceased and steps to achieve proper adhesion of the wafer  18  to the adhesion surface  12 A are repeated. After repeating the steps to achieve proper wafer adhesion, the elevation of the wafer  18  is re-measured by the height gauges  15 .  
         [0048]    On the other hand, if the respective wafer elevations as measured at the center and edge of the wafer  18  are nearly equal to each other (e.g., if the measured difference in elevation from the center to the edge of the wafer is less than 0.3 μm), then a conclusion is reached that the wafer  18  is adhered properly to the adhesion surface  12 A. In such an instance, the heat-transfer gas is introduced to the channels  23  to achieve heat dissipation from the wafer  18  to the wafer chuck  12 . Elevational measurements of the wafer  18  by the height gauges  15  are repeated as required to confirm that the wafer  18  remains attached to the adhesion surface  12 A after commencing flow of the heat-transfer gas.  
         [0049]    Since the heat-transfer gas is introduced to the channels  23  only after confirming proper adhesion of the wafer  18  to the adhesion surface  12 A of the wafer chuck  12 , as discussed above, this embodiment is effective in preventing accidental leaking of heat-transfer gas from the channels  23 .  
         [0050]    The number of height gauges  15  is not limited to two. More detailed measurements of the elevational distribution of the wafer  18  can be obtained by using more than two height gauges  15 .  
       Second Representative Embodiment  
       [0051]    This embodiment is depicted in FIG. 3. In FIG. 3, components that are similar to respective components shown and discussed in the first representative embodiment have the same respective reference numerals and are not discussed further below. Also, in FIG. 3, the adhesion-control system and the system for supplying the heat-transfer gas (see FIG. 2) are not shown. In the following description, “top,” “bottom,” “left,” and “right” correspond to top, bottom, left, and right, respectively, in FIG. 3.  
         [0052]    The following discussion of the FIG. 3 embodiment is directed mainly to the wafer chuck  12 . The wafer  18  is adhered to the adhesion surface  12 A of the wafer chuck  12  or released from the adhesion surface  12 A based on whether or not, respectively, voltage is supplied by the adhesion-control system  22  to the electrodes  12 B (see FIG. 2). Multiple (at least two) grounding pins  30  extend “upward” from the edge of the adhesion surface  12 A, such that the distal tip of each grounding pin  30  is at a higher elevation than the adhesion surface  12 A. Whenever the wafer  18  is placed on the adhesion surface  12 A of the wafer chuck  12 , the distal tips of the grounding pins  30  contact the downstream-facing surface  18 B of the wafer  18 . The grounding pins  30  desirably have a spring bias in the “upward” direction against which the downstream-facing surface  18 B of the wafer  18  is urged whenever the wafer  18  is adhered to the adhesion surface  12 A.  
         [0053]    Whenever the wafer  18  simply is resting on the adhesion surface  12 A, the contact force applied to the grounding pins  30  solely by the mass of the wafer  18  is relatively small. Consequently, the contact resistance of the grounding pins  30  to the wafer  18  is relatively high. On the other hand, whenever the wafer  18  is attracted electrostatically to the adhesion surface  12 A, the force with which the wafer  18  contacts the grounding pins  30  is relatively large. Consequently, the contact resistance of the grounding pins  30  to the wafer  18  is relatively low.  
         [0054]    The grounding pins  30 , wafer  18 , and a DC power supply  31  form a series circuit  32  in which the negative pole of the DC power supply  31  is electrically grounded. The grounding pin  30  extending from the “right” edge (in the figure) of the adhesion surface  12 A also is grounded. In contrast, the grounding pin  30  extending from the left edge (in the figure) of the adhesion surface  12 A has an electrical potential (supplied by the DC power supply  31 ) that is higher (by a specified magnitude) than the potential of the right-hand grounding pin  30 . Since the contact resistance of each grounding pin  30  to the wafer is relatively low in the series circuit  32  whenever the wafer  18 , placed on the adhesion surface  12 A, is attracted electrostatically to the adhesion surface  12 A, the current flowing through the series circuit  32  is relatively high under such conditions.  
         [0055]    A current/voltage converter  33  is connected between the “left” grounding pin  30  and the DC power supply  31 . The current flowing through the series circuit  32  is converted to a respective voltage by the current/voltage converter  33 . The respective voltage is output to a gauge  34 . The gauge  34  allows confirmation of whether the wafer  18  is adhered properly to the adhesion surface  12 A of the wafer chuck  12 .  
         [0056]    If at least one of the grounding pins  30  is not actually contacting the downstream-facing surface  18 B of the wafer  18 , then no voltage will be output from the current/voltage converter  33  to the gauge  34  because the series circuit  32  is not complete. Hence, with this embodiment, it is easy to confirm whether the downstream-facing surface  18 B of the wafer  18  is contacting each of the grounding pins  30  securely.  
         [0057]    The heat-transfer gas is introduced into the channels  23  in the adhesion surface  12 A of the wafer chuck  12  after confirming proper adhesion of the wafer  18  to the wafer chuck  12 . Consequently, gas-leakage problems accompanying poor wafer chucking are prevented.  
       Third Representative Embodiment  
       [0058]    This embodiment is described with reference to FIGS.  4 (A)- 4 (C). In FIGS.  4 (A)- 4 (C), components that are similar to respective components shown and discussed in the first representative embodiment have the same respective reference numerals and are not discussed further below.  
         [0059]    [0059]FIG. 4(A) is a schematic elevational section of a wafer  18  adhered to the wafer chuck  12 . FIG. 4(B) is a schematic elevational section of a condition in which the pressure of the heat-transfer gas in the channels  23  is excessive or the electrostatic force between the wafer  18  and wafer chuck  12  is insufficient, causing the wafer  18  to “float” on the heat-transfer gas relative to the wafer chuck  12 . FIG. 4(C) is a schematic plan view showing an exemplary arrangement of alignment pins  29  relative to the adhesion surface  12 A of the wafer chuck  12 .  
         [0060]    The alignment pins  29  serve to prevent lateral shift of the wafer  18  relative to the adhesion surface  12 A. In this embodiment, the alignment pins  29  are mounted to the sides of the wafer chuck  12  using bolts  41 . Any of various other modes of attachment of the alignment pins  29  to the wafer chuck  12  can be utilized. In this embodiment, by way of example, three alignment pins  29  are situated equi-angularly around the circumference of the wafer chuck  12 , as shown in FIG. 4(C). The alignment pins  29  desirably are made of a non-magnetic metal (e.g., copper or titanium) so as not to disturb the magnetic field around the wafer chuck  12 .  
         [0061]    By way of example, and not intending to be limiting in any way, with a wafer thickness of 1 mm, the distal (“top”) end of each alignment pin  29  is about 2-3 mm above the adhesion surface  12 A of the wafer chuck  12 . The gap between the inside edge of each alignment pin  29  and the outer edge of the wafer  18  is, e.g., 0.2-0.5 mm. Desirably, the alignment pins  29  do not actually contact the wafer  18 .  
         [0062]    In FIG. 4(A), the wafer  18  is shown as normally adhered electrostatically to the adhesion surface  12 A. In FIG. 4(B), in contrast, the electrostatic force between the wafer  18  and the wafer chuck  12  is insufficient relative to the pressure of heat-transfer gas in the channels  23 , causing the wafer  18  to float on a cushion of the heat-transfer gas. Under such a condition, the alignment pins  29  prevent the wafer from laterally shifting and falling off the wafer chuck  12 .  
         [0063]    Therefore, according to this embodiment, the substrate (wafer) to be processed is prevented from laterally shifting and/or falling off the wafer chuck, if separated from the wafer chuck, and falling off the wafer chuck.  
       Fourth Representative Embodiment  
       [0064]    This embodiment is described with reference to FIG. 5, schematically depicting (in elevational section) a wafer chamber  52 , a wafer chuck  51 , and a wafer  57 . Peripheral components are shown as a fluid-conduit system.  
         [0065]    The wafer chamber  52  and other components shown in FIG. 5 are part of a wafer-processing apparatus  50 . For example, the wafer-processing apparatus  50  can be a CPB microlithography apparatus, in which the downstream-facing surface  57 B of the wafer  57  is adhered to the adhesion surface  51 A of the wafer chuck  51  and a pattern is transferred microlithographically to the process surface  57 A of the wafer  57 . The wafer chuck  51  is situated inside the wafer chamber  52 . A turbomolecular pump  53  (as an exemplary vacuum pump) is connected to the wafer chamber  52 . The interior of the wafer chamber  52  is maintained at a high vacuum (about 1.0×10 −4  Pa) by the turbomolecular pump  53 . A vacuum gauge  54  is connected to the wafer chamber  52  and is used to monitor the vacuum level inside the wafer chamber  52 . The vacuum level detected by the vacuum gauge  54  is routed back to a flow-rate regulator  65 , described below.  
         [0066]    Channels  58  are defined in the adhesion surface  51  A of the wafer chuck  51 , except at the rim (seal)  51 B of the adhesion surface. The channels  58  are interconnected or at least contiguous with each other. The rim (seal)  51 B serves to suppress leakage of heat-transfer gas from the channels  58  into the wafer chamber  52 . A gas-inlet port  51 C passes vertically through the wafer chuck  51  and opens into a channel in the center of the adhesion surface  51 A. A gas-inlet duct  59  is connected to the gas-inlet port  51 C. Heat-transfer gas (e.g., helium gas) is introduced through the gas-inlet duct  59  to fill the channels  58  with the gas.  
         [0067]    Representative parameters used for determining thermal conductivity between the wafer chuck  51  and the wafer  57  include the thermal conductivity and pressure of the heat-transfer gas filling the channels  58  and the transverse area and profile of the channels  58 . The numerical density of atoms of the heat-transfer gas in the channels  58  is relatively low whenever the pressure of the heat-transfer gas in the channels is low. As a result, the mean free path of the gas atoms (i.e., the average distance that the atoms of gas can travel in a straight line) is significantly longer than a transverse dimension of the channel  58 . Under such conditions, the thermal conductivity of the heat-transfer gas filling the channels  58  is nearly proportional to the pressure of the gas.  
         [0068]    In contrast, the numerical density of atoms of the heat-transfer gas in the channels  58  is relatively large whenever the pressure of the heat-transfer gas in the channels  58  is high. As a result, the mean free path of the gas atoms is significantly shorter than a transverse dimension of the channel  58 . Under such conditions, the thermal conductivity of the heat-transfer gas filling the channels  58  is constant, and is not dependent on the pressure of the heat-transfer gas.  
         [0069]    The gas-inlet duct  59  originates at a flow-rate regulator  64 , which is connected via a gas-inlet duct  60  to a gas cylinder  66  containing a compressed supply of the heat-transfer gas. A vacuum gauge  55  is connected to the gas-inlet duct  59  for monitoring the pressure inside the gas-inlet duct  59 . Data concerning the pressure detected by the vacuum gauge  55  is routed back to the flow-rate regulator  64 ; hence, the flow rate of the heat-transfer gas through the gas-inlet duct  59  is controlled by the flow-rate regulator  64 . Further detail concerning this control is provided below.  
         [0070]    One or more gas-evacuation ports  51 D open into channels  58  located near the rim  51 B of the adhesion surface  51 A. The gas-evacuation ports  51 D extend through the wafer chuck  51  and are connected to a gas-evacuation conduit  61  that conducts the heat-transfer gas from the channels  58 . To evacuate the heat-transfer gas, the gas-evacuation conduit  61  is connected to a gas-evacuation duct  62  connected via a flow-rate regulator  65  and a gas-discharge duct  63  to a vacuum pump  67 . The flow rate of the heat-transfer gas in the gas-discharge duct  63  is controlled by the flow-rate regulator  65 . Further detail concerning this control is provided below.  
         [0071]    A vacuum gauge  56  is connected to the gas-evacuation duct  62  to monitor the pressure (“vacuum”) inside the gas-evacuation conduit  61  and inside the gas-evacuation duct  62 . Data concerning the pressure as measured by the vacuum gauge  56  is routed back to the flow-rate regulator  64 . In this embodiment, the pressure inside the channels  58  is estimated based on the pressure data obtained by the vacuum gauges  55 ,  56 . Whereas it is possible to consider only data from the vacuum gauge  55  in making such estimates, it is desirable to consider also the data from the vacuum gauge  56 .  
         [0072]    Flow control by the flow-rate regulators  64 ,  65  is achieved as follows. Whenever the condition of the downstream-facing surface  57 B of the wafer  57  is good (e.g., not warped and free of contaminant particles attached to the downstream-facing surface), leakage of heat-transfer gas from the channels  58  into the wafer chamber  52  can be controlled adequately by the rim (seal)  51 B of the wafer chuck  51 . Under such conditions, the leak rate of heat-transfer gas is within the evacuation-capacity range of the turbomolecular pump  53 . Consequently, a high vacuum (about 1×10 −4  Pa) can be maintained inside the wafer chamber  52  by the turbomolecular pump  53  alone. Also, whenever the condition of the downstream-facing surface  57 B of the wafer  57  is good, the flow rate of heat-transfer gas in the gas-inlet duct  59  is controlled by the flow-rate regulator  64 , and the flow rate of heat-transfer gas in the gas-evacuation duct  62  is controlled by the flow-rate regulator  65 , so as to maintain the pressure inside the channel  58  at a target value (e.g., 2.7×10 2  Pa to 1.3×10 3  Pa (2 Torr to 10 Torr) for helium). Such control ensures good thermal conductivity between the wafer chuck  51  and the wafer  57 , thereby providing good suppression of thermal expansion and deformation of the wafer.  
         [0073]    On the other hand, if the condition of the downstream-facing surface  57 B of the wafer  57  is poor, then a substantially increased leakage of heat-transfer gas from the rim (seal)  51 B of the chuck  51  would be expected. Under such conditions, the leak rate of heat-transfer gas into the wafer chamber  52  would exceed the evacuation capacity of the turbomolecular pump  53 . In extreme cases, the required high vacuum inside the wafer chamber  52  cannot be maintained by the turbomolecular pump  53  alone. Also, whenever the condition of the downstream-facing surface  57 B is poor, the flow rate of heat-transfer gas in the gas-evacuation conduit  61  and the gas-evacuation duct  62  is controlled by the flow-rate regulator  65 , so as to cause the interior of the wafer chamber  52  to return to a high vacuum (about 1.3×10 −3  Pa). Since the amount of heat-transfer gas evacuated from the channels  58  is increased by such a scheme, excessive leakage of the heat-transfer gas into the wafer chamber  52  from the channels  58  is suppressed.  
         [0074]    Incidentally, if the amount of heat-transfer gas evacuated from the channels  58  increases, then the pressure inside the channels  58  drops below the target value. Therefore, making full use of the capacity of the gas-evacuation conduit  61  and gas-evacuation duct  62 , the amount of heat-transfer gas introduced into the gas-inlet duct  59  is controlled by the flow-rate regulator  64  whenever the condition of the downstream-facing surface  57 B is poor. This keeps the thermal conductivity between the wafer chuck  51  and the wafer  57  within an acceptable tolerance. Even though initiation of this control scheme may be somewhat time-delayed, the delay can be accommodated until the pressure inside the channels  58  returns to the target value so that no adverse effect occurs on the thermal conductivity between the wafer chuck  51  and the wafer  57 .  
         [0075]    As a concrete example, assume the pressure inside the channels  58  is about 2.7×10 2  Pa whenever the channels  58  are filled with helium gas (as a representative heat-transfer gas). Assume also that the “height” (as a representative transverse dimension) of the channels  58  is 100 μm, that the mean free path of the helium atoms is 100 μm, and that the ambient temperature in the vicinity of the wafer chuck  51  is 300° K. If the condition of the downstream-facing surface  57 B of the wafer  57  is poor, then the flow rate of helium gas in the gas-inlet duct  59  can be controlled by the flow-rate regulator  64  so that the pressure inside the channels  58  is about 1.3×10 3  Pa (10 Torr).  
       Fifth Representative Embodiment  
       [0076]    A wafer chuck according to this embodiment is shown in FIGS.  6 (A)- 6 (B), wherein FIG. 6(A) is a plan view of the wafer chuck and FIG. 6(B) is an elevational section along the line A-A of FIG. 6(A). In FIGS.  6 (A)- 6 (B), components that are similar to corresponding components in the fourth representative embodiment have the same respective reference numerals and are not described further below.  
         [0077]    The wafer chuck of this embodiment comprises a first annular rim (seal)  70  and a second annular rim (seal)  71  situated radially “outside” the first annular rim  70  on the adhesion surface  51 A of the wafer chuck  51 . The annular rims  70 ,  71  perform the same function as the rim  51 B in the fourth representative embodiment. By providing the double rim  70 ,  71  in the present embodiment, leakage of heat-transfer gas from the channels  58  into the wafer chamber  52  (see FIG. 5) can be controlled even better than in the fourth representative embodiment.  
         [0078]    Multiple (e.g., eight) gas-evacuation ports  51 D′ are provided at uniform intervals in a circle in the space between the first annular rim  70  and second annular rim  71 . The gas-evacuation ports  51 D′ perform the same function as the gas-evacuation ports  51 D in the fourth representative embodiment. The gas-evacuation ports  51 D′ are connected to an annular channel  51 E inside the body of the wafer chuck  51 . Heat-transfer gas flowing through the channels  58  from the gas-inlet port  51 C collects in the gas-evacuation ports  51 D′ and is evacuated. Since the thermal conductivity of the heat-transfer gas is increased by providing multiple gas-evacuation ports  51 D′, sudden pressure changes of heat-transfer gas in the channels  58  are ameliorated. Also, the target pressure of heat-transfer gas in the channels  58  can be set lower than in the fourth representative embodiment.  
         [0079]    Although helium gas is the desired heat-transfer gas used in the representative embodiments described above, any of various other suitable gases can be used such as nitrogen. Of the various candidate heat-transfer gases, helium gas is inert and has better thermal conductivity than other candidate gases. Hence, helium is especially desirable for use as the heat-transfer gas.  
       Sixth Representative Embodiment  
       [0080]    [0080]FIG. 7 is a flow chart of steps in a process for manufacturing a microelectronic device such as a semiconductor chip (e.g., an integrated circuit or LSI device), a display panel (e.g., liquid-crystal panel), charged-coupled device (CCD), thin-film magnetic head, micromachine, for example. In step 1, the circuit for the device is designed. In step 2, a reticle (“mask”) for the circuit is manufactured. In step 2, local resizing of pattern elements can be performed to correct for proximity effects or space-charge effects during exposure. In step 3, a wafer is manufactured from a material such as silicon.  
         [0081]    Steps 4-13 are directed to wafer-processing steps, specifically “pre-process” steps. In the pre-process steps, the circuit pattern defined on the reticle is transferred onto the wafer by microlithography. Step 14 is an assembly step (also termed a “post-process” step) in which the wafer that has been passed through steps 4-13 is formed into semiconductor chips. This step can include, e.g., assembling the devices (dicing and bonding) and packaging (encapsulation of individual chips). Step 15 is an inspection step in which any of various operability and qualification tests of the device produced in step 14 are conducted. Afterward, devices that successfully pass step 15 are finished, packaged, and shipped (step 16).  
         [0082]    Steps 4-13 also provide representative details of wafer processing. Step 4 is an oxidation step for oxidizing the surface of a wafer. Step 5 involves chemical vapor deposition (CVD) for forming an insulating film on the wafer surface. Step 6 is an electrode-forming step for forming electrodes on the wafer (typically by vapor deposition). Step 7 is an ion-implantation step for implanting ions (e.g., dopant ions) into the wafer. Step 8 involves application of a resist (exposure-sensitive material) to the wafer. Step 9 involves microlithographically exposing the resist using a charged particle beam to as to imprint the resist with the reticle pattern. In step 9, a CPB microlithography apparatus as described above can be used. Step 10 involves microlithographically exposing the resist using optical microlithography. Step 11 involves developing the exposed resist on the wafer. Step 12 involves etching the wafer to remove material from areas where developed resist is absent. Step 13 involves resist separation, in which remaining resist on the wafer is removed after the etching step. By repeating steps 4-13 as required, circuit patterns as defined by successive reticles are formed superposedly on the wafer.  
         [0083]    With any of the embodiments described above, since the heat-transfer gas is introduced into the channels in the adhesion surface of the wafer chuck always after confirming that the wafer has been adhered to the wafer chuck, problems accompanying poor wafer chucking are avoided.  
         [0084]    In addition, as described above, the wafer to be processed is prevented from shifting laterally on the wafer chuck, separating from the wafer chuck, and falling off the wafer chuck, even when the wafer is floating relative to the wafer chuck on a cushion of heat-transfer gas.  
         [0085]    Furthermore, since the flow rate with which heat-transfer gas is evacuated is regulated, if the condition of the downstream-facing surface of the wafer is poor such that the detected pressure in the wafer chamber approaches or exceeds a threshold value, leakage of heat-transfer gas from the channel between the wafer and the wafer chuck into the wafer chamber nevertheless can be suppressed adequately. If flow-rate control is performed in the gas inlet while taking into consideration the pressure in the gas-evacuation conduits, the pressure inside the channels can be estimated easily.  
         [0086]    Whereas the invention has been described in connection with multiple representative embodiments, it will be understood that the invention is not limited to those embodiments. On the contrary, the invention is intended to encompass all modifications, alternatives, and equivalents as may be included within the spirit and scope of the invention, as defined by the appended claims.