Radiant energy – Inspection of solids or liquids by charged particles – Electron probe type
Reexamination Certificate
1999-12-09
2001-10-09
Berman, Jack (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
Electron probe type
Reexamination Certificate
active
06300630
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to localized vacuum processing of a workpiece and more particularly to a localized vacuum seal suitable for use in a charged particle beam system.
BACKGROUND
Localized vacuum seals are well-known for use in the semiconductor processing industry. For an example of a local vacuum seal, see Petric U.S. Pat. No. 4,607,167 incorporated herein by reference in its entirety.
Charged particle beam lithographic machines typically include within contiguous vacuum regions a beam source, beam steering and forming elements, a stage for a semiconductor wafer or a mask (the workplace) and a mechanism to move the stage with respect to the beam. For electron beam lithography, the beam source is an electron beam source and the beam steering and forming elements are an electron optical column. For focused ion beam lithography the beam source is an ion beam source and the beam steering and forming elements are ion deflection and focusing elements.
It is known from the above-referenced patent document to include what is called there an envelope apparatus coupled to the lower end of the electron beam column. The tip of the envelope apparatus rests slightly above the surface of the workpiece which is a semiconductor wafer (or a mask blank) mounted on a carrier and moving stage. Very small gaps are maintained between the tip of the envelope and the surface of the wafer. These gaps form a graded (differentially pumped) vacuum seal when the machine is in operation. Since the envelope tip is a truncated circular cone, a radial seal is thereby formed. This advantageously allows the stage supporting the wafer to be outside the vacuum. Hence a relatively small sized vacuum enclosure is possible while keeping the electron beam within the vacuum, which is necessary for its propagation.
Within the vacuum envelope, concentric conical members establish discrete zones, each zone being in communication with an associated vacuum pump. The vacuum is graded from the pressure of the ambient atmosphere, to a low vacuum level produced by the first stage vacuum pump, to a higher vacuum level formed by a second stage vacuum pump, to the vacuum along the beam path formed by a high vacuum pump in communication with the central portion of the envelope. Outside the largest seal is a ring of air bearings that are supplied with compressed air, supplied at an appropriate pressure to produce an air bearing effect between the surface of the vacuum envelope and the surface of the wafer. Typically these air bearings are air jets spaced along a circle and supplied with air from an annular groove outside the vacuum.
Related disclosure is also found in Lamattina, et al., U.S. Pat. No. 4,584,479; Petric, et al., U.S. Pat. No. 4,524,261; Petric, et al., U.S. Pat. No. 4,528,451 and Young, et al., U.S. Pat. No. 4,818,838, all incorporated herein by reference in their entireties.
However, the present inventor has identified certain deficiencies in the above-referenced vacuum seals. The above-mentioned patent documents do not appear to recognize or mention one problem introduced by their vacuum seals. This problem typically occurs during loading and unloading of the workpiece substrate (mask or wafer). In this case, the substrate, which sits on a carrier (a portion of the stage), typically sits in a recess in the carrier. Ideally the carrier is configured so that the depth of the recess is exactly equal to the thickness of the substrate, so the top surface of the substrate is exactly at the same level as (coplanar with) the upper surface of the carrier outside the recess. However, even with this arrangement, a vacuum leak develops when the gap between the edge of the substrate and the carrier traverses the vacuum seal due to movement of the stage on which the carrier rides. This is because inevitably the substrate cannot fit exactly against the corresponding edge of the recess in the carrier. During the time this gap traverses the vacuum seal, the annular regions of the seal are partially vented. In other words, the gap is a leak that introduces a large quantity of undesirable air into the vacuum portions of the seal. This of course undesirably allows the atmospheric air to enter into the electron beam column thus possibly contaminating the column, and at a minimum requiring a certain amount of time to reestablish the proper vacuum level after the gap is traversed.
SUMMARY
The above-described prior art vacuum seal is modified so that it includes an inner conical baffle that is raised a few millimeters from the substrate surface, and the outside surface of the inner conical baffle is at an intermediate vacuum of, e.g., about 10
−4
torr instead of the conventional high vacuum at 10
−6
torr. The high vacuum does not extend all the way down to the substrate. Operation of prior art environmental scanning electron microscopes has shown that one may allow the beam to pass through a short distance of intermediate vacuum (e.g., the 10
−4
torr) between the hole for the beam in the upper part of the inner conical baffle and the substrate without ill effects. In addition, there is provided an isolation valve which is, in one embodiment, flat in shape and located just above the inner conical baffle. In addition, the backscatter detector is located in the intermediate vacuum outside the inner conical baffle instead of in the high vacuum as it is in the prior art.
During loading and unloading of the substrate, the isolation valve is closed to isolate the high vacuum which is along most of the beam path inside the electron beam column. Then the substrate-carrier gap is rapidly moved through the seal region, briefly venting the intermediate vacuum regions to the ambient air or nitrogen. The small high vacuum region between the valve and inner conical baffle interior is only partially vented. When the seal is re-established on the substrate surface, the intermediate and forevacuum pumps quickly pump down to their normal (rather modest) values, even though they do not have associated isolation valves. After a brief pause, the isolation valve is opened, and the small high vacuum region between the isolation valve and inner differential pumping region is evacuated by the conventional high vacuum pumps. Since this region has a small surface area and volume, it can fairly quickly return to its vacuum value of 10
−6
torr even though it may retain some water vapor on its surfaces.
If the inner conical baffle were too large and located at the level of the substrate surface, and if there were no isolation valve, more water vapor would invade the high vacuum system along the beam path, and it would take longer to recover conditions necessary to use the beam to expose the substrate. In one embodiment, therefore, the present system does not require the isolation valve.
In one embodiment having the isolation valve, the isolation valve is located between the upper portion of the electron beam column and the inner conical baffle which is the innermost portion of the differentially pumped vacuum seal. The isolation valve is electrically linked to a control computer which controls the electron beam machine, thus allowing operation of the isolation valve in conjunction with movement of the stage on which the carrier and target substrate (workpiece) ride.
The gap between the lower portion of the vacuum seal and the surface of the substrate is maintained by the air bearings. Thus desirably the isolation valve and the associated inner conical baffle allow the gap between the substrate and the carrier to pass under the vacuum seal during loading and unloading of the substrate.
REFERENCES:
patent: 4524261 (1985-06-01), Petric et al.
patent: 4528451 (1985-07-01), Petric et al.
patent: 4560880 (1985-12-01), Petric et al.
patent: 4584479 (1986-04-01), Lamattina
patent: 4607167 (1986-08-01), Petric
patent: 5784925 (1998-07-01), Trost et al.
patent: 5838006 (1998-11-01), Veneklasen et al.
patent: 5900667 (1999-05-01), Veneklasen et al.
Berman Jack
Etec Systems, Inc.
Smith II Johnnie L
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