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Use of a Modulated Laser Beam in an Optical Focus Sensor and Control System Used in Semiconductor Mask Manufacturing

IP.com Disclosure Number: IPCOM000040312D
Original Publication Date: 1987-Oct-01
Included in the Prior Art Database: 2005-Feb-02
Document File: 2 page(s) / 44K

Publishing Venue

IBM

Related People

Burbo, JH: AUTHOR

Abstract

A modulated laser light source is incorporated in a Simon probe optical focus sensor used in the manufacture of semiconductor photomasks. Use of the modulated laser beam through the camera lens in an advanced step-and-repeat camera allows mask images to be focused within 0.25 micrometer of optimum focus. The operating principle of the focus sensor is shown in the schematic figure. The focus sensor is used to normalize the mask focal plane initially and to adjust the focus after each step of the mask camera. The laser light source 10 is modulated by an acoustic-optical modulator 12 driven by a 5 KHz oscillator 14, as shown in the figure. The modulated beam 16 is brought into focus by a lens 18. The focal spot F1 20 is imaged through the camera lens 24 at F2 which is at or near the chrome and resist-coated mask plate 26.

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Use of a Modulated Laser Beam in an Optical Focus Sensor and Control System Used in Semiconductor Mask Manufacturing

A modulated laser light source is incorporated in a Simon probe optical focus sensor used in the manufacture of semiconductor photomasks. Use of the modulated laser beam through the camera lens in an advanced step-and-repeat camera allows mask images to be focused within 0.25 micrometer of optimum focus. The operating principle of the focus sensor is shown in the schematic figure. The focus sensor is used to normalize the mask focal plane initially and to adjust the focus after each step of the mask camera. The laser light source 10 is modulated by an acoustic-optical modulator 12 driven by a 5 KHz oscillator 14, as shown in the figure. The modulated beam 16 is brought into focus by a lens
18. The focal spot F1 20 is imaged through the camera lens 24 at F2 which is at or near the chrome and resist-coated mask plate 26. The mask plate acts as a mirror, returning the laser beam through the camera lens and a beam splitter 22 to a Simon probe 28. After the beam is split by a second beam splitter 30 in the Simon probe, the divided beam is passed through pinhole apertures 32 and 34 to two detectors 36 and 38, respectively. The pinhole apertures are arranged so that one aperture 32 is 20 mm in front of focal plane F3' and the second aperture 34 is 20 mm behind focal plane F3. The detectors 36 and 38 are immediately behind apertures 32 and 34, respectively...