Image sensor comprising isolated germanium photodetectors integrated with a silicon substrate and silicon circuitry
View Patent ↗In accordance with the invention, an improved image sensor comprises an array of germanium photosensitive elements integrated with a silicon substrate and integrated with silicon readout circuits. The silicon transistors are formed first on a silicon substrate, using well known silicon wafer fabrication techniques. The germanium elements are subsequently formed overlying the silicon by epitaxial growth. The germanium elements are advantageously grown within surface openings of a dielectric cladding. Wafer fabrication techniques are applied to the elements to form isolated germanium photodiodes. Since temperatures needed for germanium processing are lower than those for silicon processing, the formation of the germanium devices need not affect the previously formed silicon devices. Insulating and metallic layers are then deposited and patterned to interconnect the silicon devices and to connect the germanium devices to the silicon circuits. The germanium elements are thus integrated to the silicon by epitaxial growth and integrated to the silicon circuitry by common metal layers.
1. An apparatus comprising:
a silicon substrate;
first and second photodetectors, each of the first and second photodetectors formed on the silicon substrate and formed of monocrystalline material comprising germanium; and
circuitry connected to the first and second photodetectors to individually address and read photoresponses of the first and second photodetectors, the circuitry comprising at least one circuit feature formed in the silicon substrate.
2. The apparatus of claim 1 , wherein the monocrystalline material of the first photodetector has a uniform concentration of germanium.
3. The apparatus of claim 2 , wherein the monocrystalline material of the first photodetector comprises at least 80% germanium.
4. The apparatus of claim 3 , wherein the monocrystalline material of the first photodetector comprises at least 90% germanium.
5. The apparatus of claim 3 , wherein the monocrystalline material of the first photodetector has a defect density less than approximately 10 3 defects/cm 3 .
6. The apparatus of claim 3 , further comprising a plurality of additional photodetectors formed on the silicon substrate in addition to the first and second photodetectors, wherein the first photodetector, second photodetector, and the plurality of additional photodetectors are arranged in an array on the silicon substrate.
7. The apparatus of claim 3 , wherein the first photodetector has a thickness between approximately 1 micron and approximately 2.3 microns.
8. The apparatus of claim 3 , wherein the silicon substrate is formed of single crystal silicon.
9. The apparatus of claim 3 , wherein the circuitry comprises a transistor disposed underneath the first photodetector, the transistor having a drain region and a source region, the drain region and the source region being formed in the silicon substrate.
10. The apparatus of claim 1 , wherein the circuitry comprising at least one circuit feature formed in the silicon substrate comprises a transistor having a source region, drain region, and channel region formed in the silicon substrate, the transistor further having a gate stack formed above the silicon substrate.