Germanium film optical device fabricated on a glass substrate
A germanium (Ge) photodiode array on a glass substrate is provided with a corresponding fabrication method. A Ge substrate is provided that is either not doped or lightly doped with a first dopant. The first dopant can be either an n or p type dopant. A first surface of the Ge substrate is moderately doped with the first dopant and bonded to a glass substrate top surface. Then, a first region of a Ge substrate second surface is heavily doped with the first dopant. A second region of the Ge substrate second surface is heavily doped with a second dopant, having the opposite electron affinity than the first dopant, forming a pn junction. An interlevel dielectric (ILD) layer is formed overlying the Ge substrate second surface and contact holes are etched in the ILD layer overlying the first and second regions of the Ge substrate second surface. The contact holes are filled with metal and metal pads are formed overlying the contact holes.
1. A method for fabricating a germanium (Ge) photodiode array on a glass substrate, the method comprising:
providing a Ge substrate selected from a group consisting of non-doped and lightly doped with a first dopant selected from a group consisting of n and p type dopants, the Ge substrate having a first surface and a second surface;
moderately doping the first surface of the Ge substrate with the first dopant;
bonding the first surface of the Ge substrate to a glass substrate top surface;
heavily doping a first region of the Ge substrate second surface with the first dopant;
heavily doping a second region of the Ge substrate second surface with a second dopant, having an opposite electron affinity than the first dopant, forming a pn junction;
forming an interlevel dielectric (ILD) layer overlying the Ge substrate second surface;
etching contact holes in the ILD layer overlying the first and second regions of the Ge substrate second surface; and,
filling the contact holes with metal and forming metal pads overlying the contact holes.
2. The method of claim 1 further comprising:
prior to bonding the Ge substrate to the glass substrate, conditioning a surface selected from a group consisting of the glass substrate top surface, the Ge substrate first surface, and both the glass substrate top surface and Ge substrate first surface.
3. The method of claim 2 wherein conditioning the selected surface includes using a process selected from a group consisting of hydrophilic, hydrophobic, and plasma treatments.
4. The method of claim 1 further comprising:
prior to bonding the Ge substrate to the glass substrate, depositing a silicon dioxide layer on a surface selected from a group consisting of the glass substrate top surface, the.Ge substrate first surface, and both the glass substrate top surface and Ge substrate first surface.
5. The method of claim 1 further comprising:
subsequent to bonding the Ge substrate to the glass substrate, thinning the Ge substrate to a thickness in a range of about 1 to 100 microns.
6. The method of claim 5 wherein thinning the Ge substrate includes using a thinning process selected from a group consisting of mechanical polishing, chemical-mechanical polishing (CMP), wet etching, and dry etching.
7. The method of claim 1 wherein moderately doping the first surface of the Ge substrate with the first dopant includes doping with a density in a range of 1 ×10 16 to 5×10 19 cm −3 .
8. The method of claim 1 wherein heavily doping the first and second regions of the Ge substrate second surface includes:
doping with a density in a range of 5×10 18 to 1×10 21 cm −3 ; and,
activating at a temperature in a range of 400 to 600 degrees C., for a duration in a range of 1 second to 10 hours.
9. The method of claim 1 wherein bonding the first surface of the Ge substrate to the glass substrate top surface includes bonding the Ge substrate to a glass substrate having a thermal expansion coefficient of about (5 to 8)×10 −6 K −1 at room temperature.
10. The method of claim 1 wherein bonding the first surface of the Ge substrate to the glass substrate top surface includes bonding at a temperature in a range of about 25 to 600 degrees C., in an environment selected from a group consisting of vacuum and ambient.
11. A method of claim 1 wherein heavily doping the first region of the Ge substrate second surface and heavily doping the second region of the Ge substrate second surface includes forming an m column by n row array of pn photodiodes;
the method further comprising:
bonding the metal pads of the pn photodiode array to electrical contacts on a silicon (Si) substrate read-out circuit having an array of pixel control elements arranged in m columns and n rows, where each pixel control element reads and resets a corresponding Ge photodiode.
12. The method of claim 11 further comprising:
subsequent to bonding the metal pads to the electrical contacts of the silicon substrate photodiode read-out circuit, removing the glass substrate.