Method and apparatus for wafer-level measurement of volume holographic gratings
View Patent ↗The invention describes a system for the measurement of volume holograms on a wafer scale that permits high-resolution and high throughput measurement of grating parameters. The invention uses a collimated beam of a fixed wavelength light source that is transmitted through the wafer to be tested. The transmitted beam is imaged with a lens onto a sensor. The sensor is used to measure the beam power under a variety of conditions, including without a wafer in place, so that the various measurements can be used to determine grating characteristics. The measurement data produced includes values for grating spacing, tilt angle, and diffraction efficiency with a high spatial resolution.
1. A system for measuring the characteristics of a volume holographic grating comprising;
a light source for generating an output beam;
a stage for mounting the holographic grating such that the volume holographic grating can be disposed in the path of the output beam and producing a transmitted beam;
a detector disposed in the path of the transmitted beam for receiving the transmitted beam and generating output values that are used to calculate characteristics of the volume holographic grating.
2. The system of claim 1 wherein the volume holographic grating is a wafer.
3. The system of claim 1 wherein the light source is a single wavelength light source.
4. The system of claim 3 wherein the single wavelength light source is a fiber coupled laser.
5. The system of claim 1 further including a collimator disposed between the light source and the volume holographic grating.
6. The system of claim 5 further including a lens disposed between the volume holographic grating and the detector.
7. The system of claim 1 wherein the detector is a 2D camera detector.
8. The system of claim 1 wherein the stage is rotatable to change the orientation of the volume holographic grating in the input beam.
9. The system of claim 1 wherein the characteristics of the volume holographic grating include diffraction efficiency, grating spacing, slant angle, and bulk transmission.
10. The system of claim 3 further including a plurality of fiber coupled lasers of different wavelengths.
11. The system of claim 10 further including fiber optic switches for selecting from the plurality of fiber coupled lasers.
12. The system of claim 11 further including focusing means for automatically modifying the focus of the system depending on the selection of one of the plurality of fiber coupled lasers.
13. The system of claim 1 further including means for removing the volume holographic grating to acquire reference measurements.
14. The system of claim 1 further including a variable attenuator to prevent overexposure of the sensor.
15. The system of claim 2 further including a carousel for holding a plurality of wafers that can be selectably placed in the path of the output beam.
16. A method of determining characteristics of a volume holographic grating comprising:
transmitting a light beam to a detector and measuring incident power;
placing the volume holographic grating in a position at a non-Bragg angle on a stage in the path of the light beam and between the light beam and the detector and measuring lost power;
placing the volume holographic grating at a number of angles including a Bragg matching condition;
measuring values of the light transmitted through the volume holographic grating onto the target and calculating characteristics of the volume holographic grating.
17. The method of claim 16 further including fitting and subtracting interference due to surface reflections.