Apparatus for etching a workpiece

[0002] 1. Field of the Invention

[0003] This invention relates to methods and apparatus for etching a workpiece using Xenon Difluoride (XeF₂). Xenon Difluoride is a dry isotropic gas phase etchant, which provides a gentle etch for silicon at low temperature. Xenon Difluoride is usually supplied in the form of colourless crystals which sublime without decomposition. The sublimation pressure for XeF₂is approximately 4 Torr.

[0004] 2. Description of the Background Art

[0005] Present attempts to use XeF₂for etching have been essentially experimental and have taken place using a pulsed supply of XeF₂which requires the etch to be stopped and started with the etch chamber being pumped down between each etch step. Such an arrangement is impracticable for production processes. Direct flow has been attempted unsuccessfully.

[0006] From one aspect the invention consists in a method of etching a workpiece using XeF₂, including allowing XeF₂in its solid phase to sublimate into its gaseous state into a reservoir of sufficient volume to provide gas at a pre-determined flow rate for a pre-determined etch period, supplying the gas at the desired flow rate to an etching chamber containing the workpiece and etching the workpiece.

[0007] The XeF₂gas may be mixed with an inert carrier gas prior to its introduction into the etch chamber. It is particularly preferred that the XeF₂source continues to sublimate during the outward flow of XeF₂from the reservoir. Additionally or alternatively the reservoir may be re-charged between the etching of separate workpieces.

[0008] From a further aspect the invention consists in apparatus for etching a workpiece including, an etched chamber, a XeF₂source, a reservoir, a valve for connecting the source to the reservoir to allow sublimation of the source into XeF₂gas, a flow controller for feeding the etch chamber and a valve for connecting the reservoir to the flow controller.

[0009] Preferably the apparatus includes pressure controlles means for maintaining the reservoir at approximately the sublimation pressure of XeF₂when there is no outward flow from the reservoir. Means may be provided for mixing the XeF₂gas with an inert carrier gas prior to its introduction into the process chamber. It is particularly preferred that is the flow controller is a pressure-based flow controller.

[0010] A chamber will normally be provided for the solid XeF₂and conveniently the reservoir may have a volume which is approximately three times the volume of the XeF chamber.

[0011] Although the invention has been described above, it is to be understood that it includes any inventive combination of the features set out above or in the following description.

[0012] The invention may be performed in various ways and a specific embodiment will be described, by way of example, with reference to the accompanying drawing, which is a schematic view of etching apparatus.

[0013] An etching apparatus is generally indicated at 10 and comprises at etch chamber 11, a XeF₂supply generally indicated at 12, a flow controller 13, a roughing pump assembly, generally indicated at 14, and a carrier gas supply 15.

[0014] The Xef₂supply comprises a XeF₂source chamber 16, which includes a tray or ampoule 17 for the XeF₂crystals 17a. The source chamber 16 is connected to a reservoir 18 via a valve 19, which in turn is connected to the flow controller 13 by a valve 20. Pressure sources 21 and 22 are provided to respectively maintain the reservoir 18 and source chamber 16 at approximately 4 Torr which is the sublimation pressure of XeF₂. Downstream of the controller 13 is a valve 23 which connects the flow controller to a supply line 24 between valves 25 and 26. Valve 25 controls the flow of the carrier gas from supply 15 into the supply is line 24, whilst valve 26 controls the supply of gases in the supply line 24 to an etch chamber 27 of the etching apparatus 11. As is conventional the roughing pump installation 14 is connected downstream of the etch chamber 27, but it is also connected to the source chamber 16 via bypass 28. A line 29 and valve 30 allows carrier gas to be supplied to this region for purging purposes.

[0015] In this the XeF₂crystals are placed in the ampoule or tray 17 with the valve 19 closed and valve 30 open. Carrier gas is used to purge the chamber and the roughing pump assembly 14 pumps the source down to the sublimation pressure. The roughing pump assembly and carrier gas are then isolated and valve 19 is opened allowing XeF₂gas to expand or diffuse into the reservoir 18.

[0016] A wafer is then loaded into the etch chamber 27 using conventional apparatus and valves 20, 23, 25 and 26 open sequentially to allow XeF₂and the carrier gas into the etch chamber where etching occurs spontaneously. The pressure within the chamber is controlled by the roughing pump assembly 14 and its automatic pressure control valve 14a. On certain occasions carrier gas may not be required in which case valve 25 remains closed.

[0017] Valve 19 may be open or closed, depending on the process and production levels which are required. An optical detector generally indicated at 31 determines when the etch has been completed or alternatively a time basis may be used. Upon completion of the etch valves 20, 23, 25 and 26 are shut and the wafer is removed. By the time a new wafer is introduced into the chamber for etching the reservoir 18 is re-charged and thus not only can each water be fully etched in one process, continuous etching of wafers is achieved. Continuous delivery of XeF₂also enhances uniformity and the use of a pressure based flow control mechanism 13 is considerably beneficial over say mass flow measurement. it will be noted that the process chamber pressure control is independent of the flow control mechanism for XeF₂.