Electrochromic tungsten oxide film deposition
A deposition method for electrochromic WO x films involves cyclic deposition of very thin poisoned and metallic tungsten oxide layers to build up a film with a desired general stoichiometry with x in the range of 3>x>2.75. The method may include: charging a deposition chamber with oxygen gas to poison a tungsten metal target; initiating sputtering of the target while reducing the oxygen partial pressure being supplied to the chamber and pumping the chamber; sputtering target for time t 1 +t 2 to form first and second tungsten oxide layers, where the first layer is deposited during time t 1 from a poisoned target and the second layer is deposited during time t 2 from a metallic target, and where the stoichiometry of the film comprising the first and second layers is a function of t 1 and t 2 ; and, repeating until a desired film thickness is achieved.
1. A method of depositing a stoichiometrically controlled tungsten oxide thin film comprising:
charging a deposition chamber with oxygen gas for time t 0 to poison the surface of a tungsten metal target with oxygen;
sputtering of said tungsten metal target and reducing the oxygen partial pressure being supplied to said deposition chamber while vacuum pumping said deposition chamber, wherein the oxygen partial pressure is gradually decreased in said deposition chamber as said tungsten metal target is sputtered;
depositing tungsten oxide sputtered from said tungsten metal target for a time t=t 1 +t 2 to form first and second tungsten oxide layers on a substrate, wherein said first tungsten oxide layer is deposited during said time t 1 from said tungsten metal target with the poisoned surface and said second tungsten oxide layer is deposited during said time t 2 from said tungsten metal target with a metallic surface, and wherein the stoichiometry of the film comprising the first and second tungsten oxide layers is a function of t 1 and t 2 ; and
repeating said charging, sputtering and depositing until a desired film thickness is achieved;
wherein there is no sputtering and no depositing during said charging.
2. The method of claim 1 , wherein the stoichiometry of the deposited film is controlled by adjusting the ratio of t 1 to t 2 .
3. The method of claim 1 , wherein the reduced oxygen partial pressure, vacuum pumping speed and sputter rate are set to transition the surface of said tungsten metal target from the oxygen poisoned state to the metallic state during a fraction of said time t.
4. The method of claim 1 , wherein said first tungsten oxide layer is WO 3 .
5. The method of claim 1 , wherein said second tungsten oxide layer is WO 2 .
6. The method of claim 1 , wherein said stoichiometry of said film is WO x , x being in the range of 2.75 to 3.
7. The method of claim 1 , wherein said substrate is at room temperature.
8. The method of claim 1 , wherein said film is less than 500 nanometers thick.
9. The method of claim 8 , wherein said charging, sputtering and depositing are repeated more than 20 times.
10. The method of claim 8 , wherein said charging, sputtering and depositing are repeated more than 30 times.
11. The method of claim 1 , wherein said sputtering is pulsed DC sputtering.
12. The method of claim 1 , wherein the oxygen partial pressure in said deposition chamber is gradually decreased by a factor of ten over said time t from a starting pressure of less than 10 mTorr.
13. The method of claim 1 , wherein the oxygen partial pressure is gradually decreased in said deposition chamber from 10 mTorr to 2 mTorr over said time t.
14. The method of claim 1 , wherein said film is optically transparent.
15. The method of claim 1 , wherein the average deposition rate of said film is greater than 5 Ångstroms per second.
16. The method as in claim 1 , wherein said charging includes flowing oxygen gas at a higher rate than during said sputtering and said depositing.
17. The method as in claim 16 , wherein said higher rate is five times higher.
18. The method as in claim 1 , further comprising, before said charging, said sputtering, said depositing and said repeating:
scanning process variables to identify a process window around the metallic/poisoned tungsten oxide transition point; and
developing a recipe including setting gas flow, deposition chamber pressure, sputter rate and times t 1 and t 2 to determine the stoichiometry of the deposited film.