Process for etching tungsten silicide overlying polysilicon particularly in a flash memory

• US 20070281477A1
• Filed: 06/02/2006
• Published: 12/06/2007
• Est. Priority Date: 06/02/2006
• Status: Active Grant

• Current Assignee: Applied Materials Incorporated
• Original Assignee: Applied Materials Incorporated
• Inventors: Lee, Kyeong-Tae, Choi, Jinhan, Jang, Bi, Shen, Meihua, Deshmukh, Shashank, Yu, Jae, Lill, Thorsten
• Granted Patent: US 7,754,610 B2
• Time in Patent Office: Days
• US Class Current: 438/689
• CPC Class Codes: H01L 21/32136 using plasmas
• CPC Class Codes: H01L 21/32137 of silicon-containing layers
• CPC Class Codes: H01L 29/40114 the electrodes comprising a conductor-insulator-conductor-insulator-semiconductor structure

Abstract

A method of plasma etching tungsten silicide over polysilicon particularly useful in fabricating flash memory having both a densely packed area and an open (iso) area requiring a long over etch due to microloading. Wafer biasing is decreased in the over etch. The principal etchant include NF₃ and Cl₂. Argon is added to prevent undercutting at the dense/iso interface. Oxygen and nitrogen oxidize any exposed silicon to increase etch selectivity and straightens the etch profile. SiCl₄ may be added for additional selectivity.

Claims

1. A method of etching a tungsten-containing layer overlying a silicon layer, comprising the steps of:

placing onto an electrode in a plasma reaction chamber a substrate containing the tungsten-containing layer and an etching mask overlying the tungsten-containing layer, wherein the silicon layer consists principally of silicon;

a first step of flowing into the chamber a first gas mixture comprising an etching gas comprising chlorine and fluorine and an oxidizing gas comprising oxygen gas and nitrogen gas;

biasing the electrode with a first level of RF power; and

applying a second level of RF power to excite the first gas mixture into a plasma to thereby etch at least the tungsten-containing layer sufficiently that at least some of the silicon layer is exposed, wherein a ratio of the second level to the first level is between 4 and 8.

2. The method of claim 1, wherein the first step further comprises flowing a noble gas selected from the group consisting of helium and argon into the chamber.

3. The method of claim 1, wherein a ratio of the nitrogen gas to the oxygen gas is at least 4.

4. The method of claim 1, wherein the second level of RF power is applied to an inductive coil associated with the chamber.

5. The method of claim 1, wherein the etching gas comprises NF₃ and Cl₂.

6. The method of claim 5, further comprising:

a subsequent second step of flowing into the chamber a second gas mixture comprising the etching gas having a smaller fraction of NF₃ than in the first step and the oxidizing gas having a larger fraction of nitrogen gas than in the first step;

biasing the electrode with a third level of RF power greater than the first level; and

applying a fourth level of RF power to excite the second gas mixture into a plasma.

7. The method of claim 6, wherein a ratio of the fourth level to the third level of RF powers is between 4 and 8.

8. The method of claim 6, further comprising the steps of:

a subsequent third step of flowing a third gas mixture into the chamber comprising the etching gas and the oxidizing gas, wherein a ratio of the nitrogen gas to the oxygen gas is greater than in the second step;

biasing the electrode with a fifth level of RF power less than the third level; and

applying a sixth level of RF power to excite the third gas mixture into a plasma

9. The method of claim 6, wherein the second gas mixture additionally comprises a passivating gas comprising silicon and chlorine constituents.

10. The method of claim 9, wherein the passivating gas comprises silicon tetrachloride.

11. The method of claim 1, wherein the first gas mixture additionally comprises a passivating gas comprising silicon and chlorine constituents.

12. The method of claim 11, wherein the passivating gas comprises silicon tetrachloride.

13. The method of claim 1, wherein the tungsten-containing layer comprises tungsten silicide.

14. The method of claim 1, wherein the silicon layer comprises polysilicon.

15. The method of claim 14, wherein the tungsten-containing layer and the silicon layer are included in a flash memory structure including an ONO layer comprising a first oxide layer, a nitride layer, and a second oxide layer overlying a gate structure formed in a silicon substrate.

16. The method of claim 15, wherein there are a plurality of such flash memory structures formed in an array in a dense area adjacent an isolation area not including such flash memory structures.

17. A method of etching a tungsten-containing layer, comprising the steps of:

placing within a plasma reaction chamber a substrate containing the tungsten-containing layer;

flowing into the plasma reaction chamber a first gas mixture comprising an etching gas comprising a chlorine- and fluorine-containing gas and a passivating gas comprising a silo-chloro compound; and

exciting the first gas mixture into a plasma.

18. The method of claim 17, wherein the tungsten-containing layer comprises tungsten silicide.

19. The method of claim 17, wherein the the silo-chloro compound comprises silicon tetrachloride.

20. The method of claim 17, wherein the etching gas comprises nitrogen trifluoride and chlorine gas.

21. The method of claim 17, wherein the first gas mixture further includes a gas chosen from the group consisting of oxygen gas and nitrogen gas.

22. The method of claim 17, wherein the exciting step includes applying RF power to an inductive coil associated with the chamber and further including supplying RF power through a capacitive coupling circuit to an electrode supporting the substrate.

23. A multi-step method of etching tungsten silicide over silicon, comprising the steps of:

placing a substrate containing a tungsten silicide layer overlying a silicon layer onto an electrode in a plasma reaction chamber;

a first step of flowing into the plasma reaction chamber a first gas mixture comprising an etching gas comprising nitrogen trifluoride and chlorine gas, an oxidizing gas comprising oxygen gas and nitrogen gas, and argon;

exciting the first gas mixture into a plasma to etch through the tungsten silicide layer in at least a portion of the substrate while biasing the electrode with a first level of RF power;

a second step of flowing into the plasma reaction chamber a second gas mixture comprising the etching gas and the oxidizing gas while biasing the electrode with a second level of RF power lower than the first level.

24. The method of claim 23, wherein less nitrogen tetrafluoride is flowed into the chamber in the second step than in the first step.

25. The method of claim 23, wherein the exciting step includes applying RF power to an inductive coil associated with the chamber.