Electricity: electrical systems and devices – Electric charge generating or conducting means – Use of forces of electric charge or field
Reexamination Certificate
2000-12-27
2003-05-27
Niebling, John F. (Department: 2812)
Electricity: electrical systems and devices
Electric charge generating or conducting means
Use of forces of electric charge or field
C324S758010
Reexamination Certificate
active
06570752
ABSTRACT:
FIELD OF THE INVENTION
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
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.
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.
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.
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.
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.
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.
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.
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
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.
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.
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 ele
Fujiwara Tomoharu
Morita Kenji
Klarquist & Sparkman, LLP
Niebling John F.
Nikon Corporation
Stevenson André C
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