Use of a Modulated Laser Beam in an Optical Focus Sensor and Control System Used in Semiconductor Mask Manufacturing
Original Publication Date: 1987-Oct-01
Included in the Prior Art Database: 2005-Feb-02
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.