And manufacturing methods of SWIR I2TUBE via heterogeneous wafer integration
A photocathode is formed with a first absorber assembly on a first substrate for a first spectral band and a second absorber assembled on a second substrate for a second spectral band. The second substrate is different from the first substrate, and the second spectral band is different from the first spectral band. The first absorber and the second absorber can be joined using a direct semiconductor wafer bonding process. The integration of a second absorber such as an SWIR absorber layer can extend the spectral response of image intensifier tubes.
1. A method of forming a photocathode, the method comprising:
(a) assembling a first absorber on a first substrate for a first spectral band;
(b) assembling a second absorber on a second substrate for a second spectral band, wherein the second substrate is different from the first substrate, and wherein the second spectral band is different from the first spectral band; and
(c) joining the first absorber and the second absorber using a direct semiconductor wafer bonding process.
2. A method according to claim 1 , wherein step (a) is practiced such that the first spectral band comprises a near infrared (NIR) wavelength, and wherein step (b) is practiced such that the second spectral band comprises a short-wave infrared (SWIR) wavelength.
3. A method according to claim 2 , wherein the first substrate comprises a GaAs substrate, and wherein step (a) comprises forming a window layer on the first substrate, and forming an NIR absorber layer on the window layer.
4. A method according to claim 2 , wherein the second substrate comprises an InP substrate, and wherein step (b) comprises forming an SWIR absorber layer on the second substrate.
5. A method according to claim 4 , further comprising tuning the second spectral band by engineering a bandgap of the SWIR absorber layer.
6. A method according to claim 2 , wherein step (a) comprises, prior to step (c), forming a first etch stop layer on the first substrate, forming a cap layer on the first etch stop layer, forming a window layer on the cap layer, and forming an NIR absorber layer on the window layer, and
wherein step (b) comprises, prior to step (c), forming a second etch stop layer on the second substrate, forming a barrier layer on the second etch stop layer, and forming an SWIR absorber layer on the barrier layer.
7. A method according to claim 6 , further comprising, after step (c), removing the first substrate and removing the first etch stop layer, thereby exposing the cap layer.
8. A method according to claim 7 , further comprising, after the removing steps, forming an anti-reflective (AR) coating layer on the cap layer.
9. A method according to claim 8 , further comprising, after forming the AR coating layer, bonding an input faceplate on the AR coating layer.
10. A method according to claim 9 , further comprising, after bonding the input faceplate, removing the second substrate and the second etch stop layer.
11. A method according to claim 6 , wherein steps (a) and (b) are practiced such that the SWIR absorber layer is thinner than the NIR absorber layer.
12. A method according to claim 1 , further comprising, after step (c), removing the first substrate to define an interim construction, bonding the interim construction to an input face plate, and subsequently removing the second substrate.
13. A photocathode formed according to the method of claim 1 .
14. A photocathode formed according to the method of claim 2 .
15. A method of forming a photocathode, the method comprising:
growing a first photocathode wafer on a GaAs substrate, the first photocathode wafer initially including a near infrared (NIR) absorber layer along with a first etch stop layer and a window layer;
growing a second photocathode wafer on an InP substrate, the second photocathode wafer initially including a short-wave infrared (SWIR) absorber layer along with a second etch stop layer and a barrier layer;
integrating the first and second photocathode wafers via a direct semiconductor wafer bonding process;
removing the first and second etch stop layers; and
bonding an input faceplate to the integrated first and second photocathode wafers.
16. A method according to claim 15 , wherein the SWIR absorber layer is thinner than the NIR absorber layer.
17. A method according to claim 15 , further comprising tuning a spectral band of the photocathode by engineering a bandgap of the SWIR absorber layer.
18. A photocathode formed according to the method of claim 15 .
19. A photocathode according to claim 17 , wherein a range of SWIR spectral response is 1.25-2.1 μm.