Atomic layer deposition of metal oxides for memory applications
Embodiments of the invention generally relate to nonvolatile memory devices and methods for manufacturing such memory devices. The methods for forming improved memory devices, such as a ReRAM cells, provide optimized, atomic layer deposition (ALD) processes for forming a metal oxide film stack which contains at least one hard metal oxide film (e.g., metal is completely oxidized or substantially oxidized) and at least one soft metal oxide film (e.g., metal is less oxidized than hard metal oxide). The soft metal oxide film is less electrically resistive than the hard metal oxide film since the soft metal oxide film is less oxidized or more metallic than the hard metal oxide film. In one example, the hard metal oxide film is formed by an ALD process utilizing ozone as the oxidizing agent while the soft metal oxide film is formed by another ALD process utilizing water vapor as the oxidizing agent.
1. A method for fabricating a resistive switching memory element, comprising:
forming a metal oxide film stack over a lower electrode disposed on a substrate, wherein the metal oxide film stack comprises a first metal oxide film and a second metal oxide film, and the metal oxide film stack is formed by:
forming the first metal oxide film during a first atomic layer deposition process, wherein the first atomic layer deposition process comprises sequentially pulsing a metal source gas and a first oxidizing agent, wherein a pulse rate of the metal source gas is greater than a pulse rate of the first oxidizing agent, and wherein the first metal oxide film substantially comprises MO 2 , where M is a metal selected from a group consisting of hafnium, zirconium, and titanium; and
after the forming of the first metal oxide film, forming the second metal oxide film on the first metal oxide film during a second atomic layer deposition process, wherein the second atomic layer deposition process comprises sequentially pulsing the metal source gas and a second oxidizing agent, wherein a pulse rate of the second oxidizing agent is greater than a pulse rate of the metal source gas, and wherein the second metal oxide film substantially comprises MO x , where M is the same metal selected in the first metal oxide film and x is within a range from about 1.70 to about 1.90;
wherein the first oxidizing agent differs from the second oxidizing agent.
2. The method of claim 1 , wherein the metal source gas and the first oxidizing agent are provided at a metal:oxygen source ratio that is greater than 1.
3. The method of claim 2 , wherein the metal:oxygen source ratio is about 60.
4. The method of claim 1 , wherein the metal source gas comprises a tetrakis(dialkylamino) hafnium compound and the first oxidizing agent comprises ozone.
5. The method of claim 1 , wherein the metal source gas comprises a tetrakis(dialkylamino) titanium compound or a tetrakis(dialkylamino) zirconium and the first oxidizing agent comprises ozone.
6. The method of claim 1 , wherein the metal source gas and the second oxidizing agent are provided at a metal:oxygen source ratio that is less than 1.
7. The method of claim 6 , wherein the metal:oxygen source ratio is about 0.25.
8. The method of claim 1 , wherein x is within a range from about 1.75 to about 1.85.
9. The method of claim 1 , wherein the first metal oxide film substantially comprises HfO 2 and the second metal oxide film substantially comprises HfO x , where x is within a range from about 1.75 to about 1.85.
10. The method of claim 1 , wherein the first metal oxide film substantially comprises ZrO 2 and the second metal oxide film substantially comprises ZrO x , where x is within a range from about 1.75 to about 1.85.
11. The method of claim 1 , wherein the first metal oxide film substantially comprises TiO 2 and the second metal oxide film substantially comprises TiO x , where x is within a range from about 1.75 to about 1.85.
12. The method of claim 1 , further comprising heating the substrate to a deposition temperature within a range from about 200° C. to about 350° C. during the first atomic layer deposition process.
13. The method of claim 1 , further comprising heating the substrate to an annealing temperature within a range from about 400° C. to about 700° C. for a time period within a range from about 2 minutes to about 10 minutes during an annealing process subsequent to the first atomic layer deposition process and prior to the second atomic layer deposition process.
14. The method of claim 1 , wherein the first metal oxide film has a current leakage at −3.0 V within a range from about 1 μA to about 100 μA.
15. The method of claim 1 , wherein the first metal oxide film has a current leakage at −0.5 V within a range from about 0.5 nA to about 10 nA.
16. The method of claim 1 , wherein the first metal oxide film has a thickness within a range from about 20 Å to about 40 Å.
17. The method of claim 1 , wherein the lower electrode comprises polysilicon.
18. A method for fabricating a resistive switching memory element, comprising:
forming a metal oxide film stack over a lower electrode disposed on a substrate, wherein the metal oxide film stack comprises a first metal oxide film and a second metal oxide film, and the metal oxide film stack is formed by:
forming the first metal oxide film over the lower electrode during a first atomic layer deposition process, wherein the first atomic layer deposition process comprises sequentially pulsing a metal source gas and an oxidizing agent, wherein a pulse rate of the metal source gas is greater than a pulse rate of the oxidizing agent, and wherein the first metal oxide film substantially comprises MO 2 , where M is a metal selected from a group consisting of hafnium, zirconium, and titanium; and
after the forming of the first metal oxide film, forming the second metal oxide film over the first metal oxide film during a second atomic layer deposition process, wherein the second atomic layer deposition process comprises sequentially pulsing the metal source gas and the oxidizing agent, wherein a pulse rate of the oxidizing agent is greater than a pulse rate of the metal source gas, and wherein the second metal oxide film substantially comprises MO x , where M is the same metal selected in the first metal oxide film and x is within a range from about 1.70 to about 1.90.
19. A method for fabricating a resistive switching memory element, comprising:
forming a metal oxide film stack over a lower electrode disposed on a substrate, wherein the metal oxide film stack comprises a metal oxide film and an additional metal oxide film, and the metal oxide film stack is formed by:
forming the metal oxide film over the lower electrode during an atomic layer deposition process, wherein the atomic layer deposition process comprises sequentially pulsing a metal source gas and a first oxidizing agent, wherein a pulse rate of the first oxidizing agent is greater than a pulse rate of the metal source gas, and wherein the metal oxide film substantially comprises MO x , where M is a metal selected from a group consisting of hafnium, zirconium, and titanium and x is within a range from about 1.70 to about 1.90; and
after the forming of the metal oxide film, forming the additional metal oxide film over the metal oxide film during a subsequent atomic layer deposition process, wherein the subsequent atomic layer deposition process comprises sequentially pulsing a metal source gas and a second oxidizing agent, wherein a pulse rate of the metal source gas is greater than a pulse rate of the second oxidizing agent, and wherein the additional metal oxide film substantially comprises MO 2 , where M is the same metal selected in the metal oxide film.