Stacked multi-spectral image sensor

A method for producing a sensing device, comprising:

• patterning a first die, which has a front side and a back side and comprises silicon, to define a first array of first sensor elements configured to output first electrical signals in response to optical radiation that is incident on the device in a first band of wavelengths less than 1000 nm that is incident on the front side of the first die;
• bonding a first side of a second die, which comprises a photosensitive material, to the back side of the first die;
• patterning the second die to define a second array of second sensor elements configured to output second electrical signals in response to the optical radiation that is incident on the device in a second band of wavelengths greater than 1000 nm that passes through the first die and is incident on the first side of the second die;
• coupling readout circuitry to read the first electrical signals and the second electrical signals serially out of the device;
• disposing a first optical filter over the front side of the first die and configured to pass both the first and second bands of wavelengths;
• disposing a second optical filter between the first and second dies and configured to pass the second band of wavelengths;
• and
• disposing multiple metal layers on the back side of the first die, including at least a first metal layer that is patterned to connect the first sensor elements to the readout circuitry and a second metal layer that is configured to serve as a part of the second optical filter,
• wherein the second metal layer is patterned to define a diffraction grating with a period chosen so as to guide the optical radiation in the second band of wavelengths toward the second sensor elements, while reflecting or deflecting other wavelengths away from the sensor elements.