US3658420A
Expired / Lapsed
US
⏱️ 0.0 years remaining
Photomask inspection by spatial filtering
Assignee: AT&T Inc
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Filing: 1969-12-10
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Granted: 1972-04-25
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Expires: 1989-12-10
Abstract
Abstract This disclosure describes an optical spatial filtering technique for detecting hole-type defects and excess spot defects in photomasks used in making microcircuits. An approximate form factor intensity filter provides suppression of the regularly shaped mask features. For masks with features whose boundaries are along only the X-Y direction, this filter advantageously is a cross placed in the transform plane. With rectangular features suppressed, only nonrectangular defect data passes. Spots as small as 0.1 mil are detected and displayed on a TV monitor; or, using a photomultiplier tube, signals are stored on an oscilloscope or by a recorder for analysis, or counted with a pulse counter. Masks or circuits on opaque substrates are also inspected by this method.
Inventors
- 👤 Norman N Axelrod
Bibliographic Details
Application Number
—
Jurisdiction
US
Data Source
google_patents
Last Synchronized
2026-10-23 11:41
CPC Classifications
G; G03; G03F; G03F1/00; G03F1/68; G03F1/82; G03F1/84; G01; G01N; G01N21/00; G01N21/84; G01N21/88; G01N21/95; G01N21/956; G01N21/95623; G02; G02B; G02B27/00; G02B27/42; G02B27/46; G06; G06V; G06V10/00; G06V10/88
Claims Analysis
Total Claims: 5
Claim 1
Independent Claim
151 words
Method for detecting defects in a photomask consisting of supposedly identical circuit elements having rectilinear geometry and located at fixed center-to-center spacing in an X-Y array, comprising the steps of: projecting a coherent collimated light beam through a rectangular slit in an opaque aperture plate and onto one or more of said circuit elements; with a transform lens, focusing the light transmitted through said mask in the transform plane of said lens; in said transform plane, interposing a spatial stop filter in the form of a cross consisting of a pair of opaque rectangular struts disposed substantially at right angles to each other, thereby to block the light diffracted by the characteristic rectilinearity of said one or more elements, but to pass the light tracing to nonrectilinear defects in said elements; detecting said passed light to produce an output; and utilizing said output to generate indicia of defects in said mask.
Claim 2
Dependent Claim
151 words
Method for detecting defects in a photomask consisting of supposedly identical circuit elements having rectilinear geometry and located at fixed center-to-center spacing in an X-Y array, comprising the steps of: projecting a coherent collimated light beam through a rectangular slit in an opaque aperture plate and onto one or more of said circuit elements; with a transform lens, focusing the light transmitted through said mask in the transform plane of said lens; in said transform plane, interposing a spatial stop filter in the form of a cross consisting of a pair of opaque rectangular struts disposed substantially at right angles to each other, thereby to block the light diffracted by the characteristic rectilinearity of said one or more elements, but to pass the light tracing to nonrectilinear defects in said elements; detecting said passed light to produce an output; and utilizing said output to generate indicia of defects in said mask.
Claim 3
Dependent Claim
31 words
Method pursuant to claim 1, comprising the further step of mechanically moving said mask in Front of said slit thereby to expose successive groups of said elements to said collimated beam.
Claim 4
Dependent Claim
137 words
Method for detecting defect in a photomask consisting of supposedly identical circuit elements having rectilinear geometry and located at fixed center-to-center spacing in an X-Y array, comprising the steps of: projecting a coherent collimated light beam onto substantially all elements of said photomask; with a transform lens, focusing the light transmitted through said mask in the transform plane of said lens; in said transform plane, interposing a spatial stop filter in the form of a cross consisting of a pair of opaque rectangular struts disposed substantially at right angles to each other, thereby to block the light diffracted by the characteristic rectilinearity of said elements, but to pass the light tracing to nonrectilinear defects in said elements; detecting said passed light to produce an output; and utilizing said output to generate indicia of defects in said mask.
Claim 5
Dependent Claim
137 words
Method for detecting defect in a photomask consisting of supposedly identical circuit elements having rectilinear geometry and located at fixed center-to-center spacing in an X-Y array, comprising the steps of: projecting a coherent collimated light beam onto substantially all elements of said photomask; with a transform lens, focusing the light transmitted through said mask in the transform plane of said lens; in said transform plane, interposing a spatial stop filter in the form of a cross consisting of a pair of opaque rectangular struts disposed substantially at right angles to each other, thereby to block the light diffracted by the characteristic rectilinearity of said elements, but to pass the light tracing to nonrectilinear defects in said elements; detecting said passed light to produce an output; and utilizing said output to generate indicia of defects in said mask.
References Cited (Backward Citations)
USPTO Annuity & Maintenance Windows
| Annuity Stage | Window Opens | Payment Deadline | Surcharge Period Until | Estimated Fee | Status |
|---|---|---|---|---|---|
| 3.5 Years (Year 4) | 1975-04-25 | 1975-10-25 | 1976-04-25 | $2000.00 | Window Closed / Lapsed |
| 7.5 Years (Year 8) | 1979-04-25 | 1979-10-25 | 1980-04-25 | $3760.00 | Window Closed / Lapsed |
| 11.5 Years (Year 12) | 1983-04-25 | 1983-10-25 | 1984-04-25 | $7700.00 | Window Closed / Lapsed |