AZA-POLYSILANE PRECURSORS AND METHODS FOR DEPOSITING FILMS COMPRISING SAME

• US 20150147871A1
• Filed: 06/02/2014
• Published: 05/28/2015
• Est. Priority Date: 06/26/2013
• Status: Active Grant

• Current Assignee: Versum Materials US Inc.
• Original Assignee: Air Products and Chemicals Incorporated (Linde Plc)
• Inventors: Xiao, Manchao, Lei, Xinjian, Spence, Daniel P.
• Granted Patent: US 9,796,739 B2
• Time in Patent Office: Days
• US Class Current: 438/482
• CPC Class Codes: C07F 7/025 without C-silicon linkages
• CPC Class Codes: C07F 7/10 containing nitrogen having a Si-N linkage
• CPC Class Codes: C07F 7/21 Cyclic compounds having at least one ring containing silicon, but no carbon in the ring
• CPC Class Codes: C23C 16/24 Deposition of silicon only
• CPC Class Codes: C23C 16/345 Silicon nitride
• CPC Class Codes: C23C 16/401 containing silicon
• CPC Class Codes: C23C 16/45544 characterized by the apparatus
• CPC Class Codes: C23C 16/45553 characterized by the use of precursors specially adapted for ALD
• CPC Class Codes: H01L 21/02126 the material containing Si, O, and at least one of H, N, C, F, or other non-metal elements, e.g. SiOC, SiOC:H or SiONC
• CPC Class Codes: H01L 21/02164 the material being a silicon oxide, e.g. SiO2
• CPC Class Codes: H01L 21/02167 the material being a silicon carbide not containing oxygen, e.g. SiC, SiC:H or silicon carbonitrides H01L21/02126 and H01L21/0214 take precedence
• CPC Class Codes: H01L 21/0217 the material being a silicon nitride not containing oxygen, e.g. SixNy or SixByNz H01L21/02126 and H01L21/0214 take precedence
• CPC Class Codes: H01L 21/02274 in the presence of a plasma [PECVD]
• CPC Class Codes: H01L 21/0228 deposition by cyclic CVD, e.g. ALD, ALE, pulsed CVD
• CPC Class Codes: H01L 21/02532 Silicon, silicon germanium, germanium
• CPC Class Codes: H01L 21/02592 amorphous
• CPC Class Codes: H01L 21/0262 Reduction or decomposition of gaseous compounds, e.g. CVD

Abstract

Described herein are precursors and methods for forming silicon-containing films. In one aspect, there is provided an aza-polysilane precursor comprising at least two Si—N bonds, at least one Si—Si bond, and at least two SiH₂ groups represented by the following Formula IA, IB and IC:

wherein R¹ and R² are independently selected from a linear or branched C₁ to C₁₀ alkyl group, a linear or branched C₃ to C₁₀ alkenyl group, a linear or branched C₃ to C₁₀ alkynyl group, C₃ to C₁₀ cyclic alkyl group, C₃ to C₁₀ hetero-cyclic alkyl group, a C₅ to C₁₀ aryl group, and a C₃ to C₁₀ hetero-aryl group, a C₂ to C₁₀ dialkylamino group, a C₃ to C₁₀ cyclic alkylamino group; R³ and R⁴ are independently selected from hydrogen, a linear or branched C₁ to C₁₀ alkyl group, a linear or branched C₂ to C₁₀ alkenyl group, a linear or branched C₂ to C₁₀ alkynyl group, C₃ to C₁₀ cyclic alkyl group, C₃ to C₁₀ hetero-cyclic alkyl group, a C₅ to C₁₀ aryl group, and a C₃ to C₁₀ hetero-aryl group, a C₂ to C₁₀ dialkylamino group, a C₃ to C₁₀ cyclic alkylamino group; wherein R¹ in Formula IA cannot both be methyl, R¹ and R² in Formula IB cannot both be iso-propyl, tert-butyl, and bezenyl and R³ and R⁴ cannot both be methyl and phenyl.

Claims

1. An aza-polysilane precursor comprising at least two Si—

N bonds, at least one Si—

Si bond, and an at least two SiH₂ groups represented by the following Formulae IA, IB and IC;

2. The aza-polysilane precursor of claim 1 selected from the group consisting of 3-iso-propyl-3-aza-pentasilane, 3-tert-butyl-3-aza-pentasilane, 3-tert-pentyl-3-aza-pentasilane, 3-cyclopentyl-3-aza-pentasilane, 3-cyclohexyl-3-aza-pentasilane, 3-(2,6-dim ethylcyclohexyl-3-aza-pentasilane, 3-(tetrahydropyran-4-yl)-3-aza-pentasilane, 3-(1-methylpiperidin-4-yl)-3-aza-pentasilane, 3-phenyl-3-aza-pentasilane, 3-(2-methyl-tolyl)-3-aza-pentasilane, 3-(2,6-dimethyl-tolyl)-3-aza-pentasilane, 3-(pyridin-3-yl)-3-aza-pentasilane, 3-(4-methylpyridin-3-yl)-3-aza-pentasilane 1,4-bis(cyclopentyl)-1,4-diaza-2,3,5,6-tetrasilacyclohexane, 1,4-bis(cyclohexyl)-1,4-diaza-2,3,5,6-tetrasilacyclohexane, and 1,4-bis(2,6-dimethylcyclohexyl)-1,4-Diaza-2,3,5,6-tetrasilacyclohexane.

3. A composition comprising:

(a) at least one aza-polysilane precursor comprising at least two Si—

N bonds, at least one Si—

Si bond, and at least two SiH₂ groups represented by the following Formulae IA, IB, and IC;

4. The aza-polysilane precursor of claim 3 comprising at least one selected from the group consisting of 3-iso-propyl-3-aza-pentasilane, 3-tert-butyl-3-aza-pentasilane, 3-tert-pentyl-3-aza-pentasilane, 3-cyclopentyl-3-aza-pentasilane, 3-cyclohexyl-3-aza-pentasilane, 3-(2,6-dim ethylcyclohexyl-3-aza-pentasilane, 3-(tetrahydropyran-4-yl)-3-aza-pentasilane, 3-(1-methylpiperidin-4-yl)-3-aza-pentasilane, 3-phenyl-3-aza-pentasilane, 3-(2-methyl-tolyl)-3-aza-pentasilane, 3-(2,6-dimethyl-tolyl)-3-aza-pentasilane, 3-(pyridin-3-yl)-3-aza-pentasilane, 3-(4-methylpyridin-3-yl)-3-aza-pentasilane 1,4-bis(cyclopentyl)-1,4-diaza-2,3,5,6-tetrasilacyclohexane, 1,4-bis(cyclohexyl)-1,4-diaza-2,3,5,6-tetrasilacyclohexane, 1,4-bis(2,6-dimethylcyclohexyl)-1,4-Diaza-2,3,5,6-tetrasilacyclohexane, 1,4-bis(iso-propyl)-1,4-diaza-2,3,5,6-tetrasilacyclohexane, 1,4-bis(tert-butyl)-1,4-diaza-2,3,5,6-tetrasilacyclohexane, 1,4-bis(tert-pentyl)-1,4-diaza-2,3,5,6-tetrasilacyclohexane.

5. The solvent of claim 3 comprising at least one selected from the group consisting of ether, tertiary amine, alkyl hydrocarbon, aromatic hydrocarbon, tertiary aminoether.

6. A method for forming a silicon-containing film on at least one surface of a substrate by a deposition process selected from a chemical vapor deposition process and an atomic layer deposition process, the method comprising:

providing the at least one surface of the substrate in a reaction chamber;

introducing at least one aza-polysilane precursor comprising at least two Si—

N bonds, at least one Si—

Si bond, and at least two SiH₂ groups represented by the following Formulae IA, IB, and IC;

7. The method of claim 6 wherein the at least one aza-polysilane precursor is selected from the group consisting of 3-iso-propyl-3-aza-pentasilane, 3-tert-butyl-3-aza-pentasilane, 3-tert-pentyl-3-aza-pentasilane, 3-cyclopentyl-3-aza-pentasilane, 3-cyclohexyl-3-aza-pentasilane, 3-(2,6-dimethylcyclohexyl-3-aza-pentasilane, 3-(tetrahydropyran-4-yl)-3-aza-pentasilane, 3-(1-methylpiperidin-4-yl)-3-aza-pentasilane, 3-phenyl-3-aza-pentasilane, 3-(2-methyl-tolyl)-3-aza-pentasilane, 3-(2,6-dimethyl-tolyl)-3-aza-pentasilane, 3-(pyridin-3-yl)-3-aza-pentasilane, 3-(4-methylpyridin-3-yl)-3-aza-pentasilane 1,4-bis(cyclopentyl)-1,4-diaza-2,3,5,6-tetrasilacyclohexane, 1,4-bis(cyclohexyl)-1,4-diaza-2,3,5,6-tetrasilacyclohexane, 1,4-bis(2,6-dimethylcyclohexyl)-1,4-Diaza-2,3,5,6-tetrasilacyclohexane, 1,4-bis(iso-propyl)-1,4-diaza-2,3,5,6-tetrasilacyclohexane, 1,4-bis(tert-butyl)-1,4-diaza-2,3,5,6-tetrasilacyclohexane, 1,4-bis(tert-pentyl)-1,4-diaza-2,3,5,6-tetrasilacyclohexane.

8. The method of claim 6 wherein the nitrogen-containing source is selected from the group consisting of ammonia, hydrazine, monoalkylhydrazine, dialkylhydrazine, nitrogen, nitrogen/hydrogen, ammonia plasma, nitrogen plasma, nitrogen/hydrogen plasma, nitrogen/helium plasma, nitrogen/argon plasma, helium plasma, argon plasma, hydrogen plasma, and mixtures thereof.

9. The method of claim 6 wherein the silicon-containing film is selected from the group consisting of silicon nitride and silicon carbonitride.

10. A method of forming a silicon-containing film via an atomic layer deposition (ALD) process, the method comprising the steps of:

a. providing a substrate in an ALD reactor;

b. providing in the ALD reactor at least one aza-polysilane precursor comprising at least two Si—

N bonds, at least one Si—

Si bond, and at least two SiH₂ groups represented by the following Formulae IA, IB, and IC;

11. The method of claim 10 wherein the at least one aza-polysilane precursor is selected from the group consisting of 3-iso-propyl-3-aza-pentasilane, 3-tert-butyl-3-aza-pentasilane, 3-tert-pentyl-3-aza-pentasilane, 3-cyclopentyl-3-aza-pentasilane, 3-cyclohexyl-3-aza-pentasilane, 3-(2,6-dimethylcyclohexyl-3-aza-pentasilane, 3-(tetrahydropyran-4-yl)-3-aza-pentasilane, 3-(1-methylpiperidin-4-yl)-3-aza-pentasilane, 3-phenyl-3-aza-pentasilane, 3-(2-methyl-tolyl)-3-aza-pentasilane, 3-(2,6-dimethyl-tolyl)-3-aza-pentasilane, 3-(pyridin-3-yl)-3-aza-pentasilane, 3-(4-methylpyridin-3-yl)-3-aza-pentasilane 1,4-bis(cyclopentyl)-1,4-diaza-2,3,5,6-tetrasilacyclohexane, 1,4-bis(cyclohexyl)-1,4-diaza-2,3,5,6-tetrasilacyclohexane, 1,4-bis(2,6-dimethylcyclohexyl)-1,4-Diaza-2,3,5,6-tetrasilacyclohexane, 1,4-bis(iso-propyl)-1,4-diaza-2,3,5,6-tetrasilacyclohexane, 1,4-bis(tert-butyl)-1,4-diaza-2,3,5,6-tetrasilacyclohexane, 1,4-bis(tert-pentyl)-1,4-diaza-2,3,5,6-tetrasilacyclohexane.

12. The method of claim 10 wherein the nitrogen-containing source is selected from the group consisting of ammonia, hydrazine, monoalkylhydrazine, dialkylhydrazine, nitrogen, nitrogen/hydrogen, ammonia plasma, nitrogen plasma, nitrogen/hydrogen plasma, nitrogen/helium plasma, nitrogen/argon plasma, helium plasma, argon plasma, hydrogen plasma, and mixtures thereof.

13. The method of claim 10 wherein the silicon-containing film is selected from the group consisting of silicon nitride and silicon carbonitride.

14. A method of forming a silicon-containing film onto at least a surface of a substrate using a deposition process selected from a plasma enhanced atomic layer (PEALD) process and a PECCVD process, the method comprising:

a. providing a substrate in an ALD reactor;

b. providing in the ALD reactor at least one aza-polysilane precursor comprising at least two Si—

N bonds, at least one Si—

Si bond, and at least two SiH₂ groups represented by the following Formulae IA, IB, and IC;

15. The method of claim 14 wherein the at least one aza-polysilane precursor is selected from the group consisting of 3-iso-propyl-3-aza-pentasilane, 3-tert-butyl-3-aza-pentasilane, 3-tert-pentyl-3-aza-pentasilane, 3-cyclopentyl-3-aza-pentasilane, 3-cyclohexyl-3-aza-pentasilane, 3-(2,6-dimethylcyclohexyl-3-aza-pentasilane, 3-(tetrahydropyran-4-yl)-3-aza-pentasilane, 3-(1-methylpiperidin-4-yl)-3-aza-pentasilane, 3-phenyl-3-aza-pentasilane, 3-(2-methyl-tolyl)-3-aza-pentasilane, 3-(2,6-dimethyl-tolyl)-3-aza-pentasilane, 3-(pyridin-3-yl)-3-aza-pentasilane, 3-(4-methylpyridin-3-yl)-3-aza-pentasilane 1,4-bis(cyclopentyl)-1,4-diaza-2,3,5,6-tetrasilacyclohexane, 1,4-bis(cyclohexyl)-1,4-diaza-2,3,5,6-tetrasilacyclohexane, 1,4-bis(2,6-dimethylcyclohexyl)-1,4-Diaza-2,3,5,6-tetrasilacyclohexane, 1,4-bis(iso-propyl)-1,4-diaza-2,3,5,6-tetrasilacyclohexane, 1,4-bis(tert-butyl)-1,4-diaza-2,3,5,6-tetrasilacyclohexane, 1,4-bis(tert-pentyl)-1,4-diaza-2,3,5,6-tetrasilacyclohexane.

16. The method of claim 14 wherein the nitrogen-containing source is selected from the group consisting of ammonia, hydrazine, monoalkylhydrazine, dialkylhydrazine, nitrogen, nitrogen/hydrogen, ammonia plasma, nitrogen plasma, nitrogen/hydrogen plasma, nitrogen/helium plasma, nitrogen/argon plasma, helium plasma, argon plasma, hydrogen plasma, and mixtures thereof.

17. The method of claim 14 wherein the silicon-containing film is selected from the group consisting of silicon nitride and silicon carbonitride.

18. A method for forming a silicon oxide or a carbon doped silicon oxide film on a substrate comprising:

reacting an oxygen-containing source with a precursor comprising at least one aza-polysilane precursor comprising at least two Si—

N bonds, at least one Si—

Si bond, and at least two SiH₂ groups represented by the following Formulae IA, IB, and IC;

19. The method of claim 18 wherein the vapor deposition is at least one selected from the group consisting of at least one selected from chemical vapor deposition, low pressure vapor deposition, plasma enhanced chemical vapor deposition, cyclic chemical vapor deposition, plasma enhanced cyclic chemical vapor deposition, atomic layer deposition, and plasma enhanced atomic layer deposition.

20. The method of claim 18 wherein the at least one aza-polysilane precursor is selected from the group consisting of 3-iso-propyl-3-aza-pentasilane, 3-tert-butyl-3-aza-pentasilane, 3-tert-pentyl-3-aza-pentasilane, 3-cyclopentyl-3-aza-pentasilane, 3-cyclohexyl-3-aza-pentasilane, 3-(2,6-dimethylcyclohexyl-3-aza-pentasilane, 3-(tetrahydropyran-4-yl)-3-aza-pentasilane, 3-(1-methylpiperidin-4-yl)-3-aza-pentasilane, 3-phenyl-3-aza-pentasilane, 3-(2-methyl-tolyl)-3-aza-pentasilane, 3-(2,6-dimethyl-tolyl)-3-aza-pentasilane, 3-(pyridin-3-yl)-3-aza-pentasilane, 3-(4-methylpyridin-3-yl)-3-aza-pentasilane 1,4-bis(cyclopentyl)-1,4-diaza-2,3,5,6-tetrasilacyclohexane, 1,4-bis(cyclohexyl)-1,4-diaza-2,3,5,6-tetrasilacyclohexane, 1,4-bis(2,6-dimethylcyclohexyl)-1,4-Diaza-2,3,5,6-tetrasilacyclohexane, 1,4-bis(iso-propyl)-1,4-diaza-2,3,5,6-tetrasilacyclohexane, 1,4-bis(tert-butyl)-1,4-diaza-2,3,5,6-tetrasilacyclohexane, and 1,4-bis(tert-pentyl)-1,4-diaza-2,3,5,6-tetrasilacyclohexane.

21. The method of claim 18 wherein the reacting step is conducted at a temperature of 200°

C. or less.

22. The method of claim 18 wherein the reacting step is conducted at a temperature of 100°

C. or less.

23. The method of claim 18 wherein the reacting step is conducted at 50°

C. or less.

24. A method for forming a silicon oxide or carbon doped silicon oxide film on a substrate comprising:

forming via vapor deposition the film on the substrate from a composition comprising at least one aza-polysilane precursor comprising at least two Si—

N bonds, at least one Si—

Si bond, and at least two SiH₂ groups represented by the following Formulae IA, IB, and IC;

25. The method of claim 24 wherein the at least one aza-polysilane precursor is selected from the group consisting of 3-iso-propyl-3-aza-pentasilane, 3-tert-butyl-3-aza-pentasilane, 3-tert-pentyl-3-aza-pentasilane, 3-cyclopentyl-3-aza-pentasilane, 3-cyclohexyl-3-aza-pentasilane, 3-(2,6-dimethylcyclohexyl-3-aza-pentasilane, 3-(tetrahydropyran-4-yl)-3-aza-pentasilane, 3-(1-methylpiperidin-4-yl)-3-aza-pentasilane, 3-phenyl-3-aza-pentasilane, 3-(2-methyl-tolyl)-3-aza-pentasilane, 3-(2,6-dimethyl-tolyl)-3-aza-pentasilane, 3-(pyridin-3-yl)-3-aza-pentasilane, 3-(4-methylpyridin-3-yl)-3-aza-pentasilane 1,4-bis(cyclopentyl)-1,4-diaza-2,3,5,6-tetrasilacyclohexane, 1,4-bis(cyclohexyl)-1,4-diaza-2,3,5,6-tetrasilacyclohexane, 1,4-bis(2,6-dimethylcyclohexyl)-1,4-Diaza-2,3,5,6-tetrasilacyclohexane, 1,4-bis(iso-propyl)-1,4-diaza-2,3,5,6-tetrasilacyclohexane, 1,4-bis(tert-butyl)-1,4-diaza-2,3,5,6-tetrasilacyclohexane, 1,4-bis(tert-pentyl)-1,4-diaza-2,3,5,6-tetrasilacyclohexane.

26. The method of claim 24 wherein the forming step is conducted at a temperature of 200°

C. or less.

27. The method of claim 24 wherein the forming step is conducted at a temperature of 100°

C. or less.

28. The method of claim 24 wherein the forming step is conducted at 50°

C. or less.

29. A method for forming a silicon oxide or carbon doped silicon oxide film on a substrate comprising:

introducing at least one aza-polysilane precursor comprising at least two Si—

N bonds, at least one Si—

Si bond, and at least two SiH₂ groups represented by the following Formulae IA, IB, and IC;

30. A method for forming a silicon oxide or carbon doped silicon oxide film on a substrate wherein the film comprises a thickness, the method comprising:

a. introducing at least one aza-polysilane precursor comprising at least two Si—

N bonds, a Si—

Si bond, and at least two SiH₂ groups represented by the following Formulae IA, IB, and IC;

31. The method of claim 30 wherein steps a. through d. and optional step e. are repeated until the thickness of film is established.

32. The method of claim 30 wherein the at least one aza-polysilane precursor is selected from the group consisting of 3-iso-propyl-3-aza-pentasilane, 3-tert-butyl-3-aza-pentasilane, 3-tert-pentyl-3-aza-pentasilane, 3-cyclopentyl-3-aza-pentasilane, 3-cyclohexyl-3-aza-pentasilane, 3-(2,6-dimethylcyclohexyl-3-aza-pentasilane, 3-(tetrahydropyran-4-yl)-3-aza-pentasilane, 3-(1-methylpiperidin-4-yl)-3-aza-pentasilane, 3-phenyl-3-aza-pentasilane, 3-(2-methyl-tolyl)-3-aza-pentasilane, 3-(2,6-dimethyl-tolyl)-3-aza-pentasilane, 3-(pyridin-3-yl)-3-aza-pentasilane, 3-(4-methylpyridin-3-yl)-3-aza-pentasilane 1,4-bis(cyclopentyl)-1,4-diaza-2,3,5,6-tetrasilacyclohexane, 1,4-bis(cyclohexyl)-1,4-diaza-2,3,5,6-tetrasilacyclohexane, 1,4-bis(2,6-dimethylcyclohexyl)-1,4-Diaza-2,3,5,6-tetrasilacyclohexane, 1,4-bis(iso-propyl)-1,4-diaza-2,3,5,6-tetrasilacyclohexane, 1,4-bis(tert-butyl)-1,4-diaza-2,3,5,6-tetrasilacyclohexane, 1,4-bis(tert-pentyl)-1,4-diaza-2,3,5,6-tetrasilacyclohexane.

33. The method of claim 30 wherein the chemisorbing step is conducted at a temperature of 200°

C. or less.

34. The method of claim 30 wherein the chemisorbing step is conducted at a temperature of 100°

C. or less.

35. The method of claim 30 wherein the chemisorbing step is conducted at 50°

C. or less.

36. The method of claim 30 is an atomic layer deposition process.

37. The method of claim 30 is a plasma enhanced cyclic chemical vapor deposition process.

38. A method for forming a silicon containing film using a deposition method selected from ALD or cyclic CVD that comprises the steps of:

a. placing a substrates into a reactor which is heated to one or more temperatures ranging from about ambient temperature to about 700°

C.;

b. introducing at least one aza-polysilane precursor comprising at least two Si—

N bonds, at least one Si—

Si bond, and at least two SiH₂ groups represented by the following Formula IA, IB, IC;

39. The method of claim 38 wherein the reducing agent is at least one selected from the group consisting of hydrogen, hydrogen plasma, helium plasma, argon plasma, or hydrogen chloride.

40. A method of depositing an amorphous or a crystalline silicon film via a deposition process selected from an atomic layer deposition, a cyclic chemical vapor deposition process and a chemical vapor deposition, the method comprising the steps of:

a. providing a substrate in a reactor;

b. introducing into the reactor at least one aza-polysilane precursor comprising at least two Si—

N bonds, at least one Si—

Si bond, and at least two SiH₂ groups represented by the following Formulae IA, IB, and IC;

41. The method of claim 40 wherein the at least one aza-polysilane precursor is selected from the group consisting of 3-iso-propyl-3-aza-pentasilane, 3-tert-butyl-3-aza-pentasilane, 3-tert-pentyl-3-aza-pentasilane, 3-cyclopentyl-3-aza-pentasilane, 3-cyclohexyl-3-aza-pentasilane, 3-(2,6-dimethylcyclohexyl-3-aza-pentasilane, 3-(tetrahydropyran-4-yl)-3-aza-pentasilane, 3-(1-methylpiperidin-4-yl)-3-aza-pentasilane, 3-phenyl-3-aza-pentasilane, 3-(2-methyl-tolyl)-3-aza-pentasilane, 3-(2,6-dimethyl-tolyl)-3-aza-pentasilane, 3-(pyridin-3-yl)-3-aza-pentasilane, 3-(4-methylpyridin-3-yl)-3-aza-pentasilane 1,4-bis(cyclopentyl)-1,4-diaza-2,3,5,6-tetrasilacyclohexane, 1,4-bis(cyclohexyl)-1,4-diaza-2,3,5,6-tetrasilacyclohexane, 1,4-bis(2,6-dimethylcyclohexyl)-1,4-Diaza-2,3,5,6-tetrasilacyclohexane, 1,4-bis(iso-propyl)-1,4-diaza-2,3,5,6-tetrasilacyclohexane, 1,4-bis(tert-butyl)-1,4-diaza-2,3,5,6-tetrasilacyclohexane, 1,4-bis(tert-pentyl)-1,4-diaza-2,3,5,6-tetrasilacyclohexane.

42. A vessel which is used to deliver a precursor for the deposition of a silicon-containing film, the vessel comprising:

at least one aza-polysilane precursor comprising at least two Si—

N bonds, at least one Si—

Si bond, and at least two SiH₂ groups represented by the following Formula IA, IB, and IC;

43. The vessel of claim 42 wherein the vessel is comprised of stainless steel.