HALOGENATED ORGANOAMINOSILANE PRECURSORS AND METHODS FOR DEPOSITING FILMS COMPRISING SAME

• US 20130078392A1
• Filed: 09/18/2012
• Published: 03/28/2013
• Est. Priority Date: 09/27/2011
• Status: Active Grant

• Current Assignee: Versum Materials US Inc.
• Original Assignee: Air Products and Chemicals Incorporated (Linde Plc)
• Inventors: Xiao, Manchao, Lei, Xinjian, O'Neill, Mark Leonard, Pearlstein, Ronald Martin, Bowen, Heather Regina, Derecskei-Kovacs, Agnes, Han, Bing, Chandra, Haripin
• Granted Patent: US 8,993,072 B2
• Time in Patent Office: Days
• US Class Current: 427/579
• CPC Class Codes: C07F 7/025 without C-silicon linkages
• CPC Class Codes: C07F 7/12 Organo silicon halides
• CPC Class Codes: C23C 16/402 Silicon dioxide
• CPC Class Codes: C23C 16/45536 Use of plasma, radiation or electromagnetic fields
• 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 21/02219 the compound comprising silicon and nitrogen
• 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 21/0262 Reduction or decomposition of gaseous compounds, e.g. CVD

Abstract

Described herein are precursors and methods of forming films. In one aspect, there is provided a precursor having Formula I:

X_mR¹_nH_pSi(NR²R³)_4-m-n-p  I

wherein X is selected from Cl, Br, I; R¹ is selected from linear or branched C₁-C₁₀ alkyl group, a C₂-C₁₂ alkenyl group, a C₂-C₁₂ alkynyl group, a C₄-C₁₀ cyclic alkyl, and a C₆-C₁₀ aryl group; R² is selected from a linear or branched C₁-C₁₀ alkyl, a C₃-C₁₂ alkenyl group, a C₃-C₁₂ alkynyl group, a C₄-C₁₀ cyclic alkyl group, and a C₆-C₁₀ aryl group; R³ is selected from a branched C₃-C₁₀ alkyl group, a C₃-C₁₂ alkenyl group, a C₃-C₁₂ alkynyl group, a C₄-C₁₀ cyclic alkyl group, and a C₆-C₁₀ aryl group; m is 1 or 2; n is 0, 1, or 2; p is 0, 1 or 2; and m+n+p is less than 4, wherein R² and R³ are linked or not linked to form a ring.

Claims

1. A halogenated organoaminosilane precursor represented by the following Formula I:

X_mR¹_nH_pSi(NR²R³)_4-m-n-p 

 

I

wherein X is a halide selected from the group consisting of Cl, Br, I;

R¹ is independently selected from a linear or branched C₁ to C₁₀ alkyl group, a C₂ to C₁₂ alkenyl group, a C₂ to C₁₂ alkynyl group, a C₄ to C₁₀ cyclic alkyl group, and a C₆ to C₁₀ aryl group;

R² is selected from a linear or branched C₁ to C₁₀ alkyl, a C₃ to C₁₂ alkenyl group, a C₃ to C₁₂ alkynyl group, a C₄ to C₁₀ cyclic alkyl group, and a C₆ to C₁₀ aryl group;

R³ is selected from a branched C₃ to C₁₀ alkyl group, a C₃ to C₁₂ alkenyl group, a C₃ to C₁₂ alkynyl group, a C₄ to C₁₀ cyclic alkyl group, and a C₆ to C₁₀ aryl group;

m is 1 or 2;

n is 0, 1, or 2;

p is 0, 1 or 2; and

the sum of (m+n+p) is less than 4 and wherein R² and R³ are linked to form a ring.

2. The halogenated organoaminosilane precursor of claim 1 wherein R¹ and R² are the same.

3. The halogenated organoaminosilane precursor of claim 1 wherein R¹ and R² are different.

4. A method for forming a dielectric film on at least one surface of a substrate by a deposition process chosen 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 halogenated organoaminosilane precursor having the following Formula I;

X_mR¹_nH_pSi(NR²R³)_4-m-n-p 

 

I

wherein X is a halide selected from the group consisting of Cl, Br, I;

R¹ is independently selected from a linear or branched C₁ to C₁₀ alkyl group, a C₂ to C₁₂ alkenyl group, a C₂ to C₁₂ alkynyl group, a C₄ to C₁₀ cyclic alkyl, and a C₆ to C₁₀ aryl group;

R² is selected from a linear or branched C₁ to C₁₀ alkyl group, a C₃ to C₁₂ alkenyl group, a C₃ to C₁₂ alkynyl group, a C₄ to C₁₀ cyclic alkyl group, and a C₆ to C₁₀ aryl group;

R³ is selected from a branched C₃ to C₁₀ alkyl group, a C₃ to C₁₂ alkenyl group, a C₃ to C₁₂ alkynyl group, a C₄ to C₁₀ cyclic alkyl group, and a C₆ to C₁₀ aryl group;

m is 1 or 2;

n is 0, 1, or 2;

p is 0, 1 or 2; and

the sum of (m+n+p) is less than 4 and wherein R² and R³ are linked to form a ring;

introducing a nitrogen-containing source into the reactor wherein the at least one organoaminosilane precursor and the nitrogen-containing source react to the dielectric films on the at least one surface.

5. The method of claim 4 wherein the at least one organoaminosilane precursor is selected from the group consisting of 2,6-dimethylpiperidinodichlorosilane, 2,6-dimethylpiperidinochlorosilane, cyclohexylmethylaminochlorosilane, cyclohexylethylaminochlorosilane, and cyclohexyl-iso-propylaminochlorosilane.

6. The method of claim 5 wherein the at least one organoaminosilane precursor comprises 2,6-dimethylpiperidinodichlorosilane.

7. The method of claim 5 wherein the at least one organoaminosilane precursor comprises 2,6-dimethylpiperidinochlorosilane.

8. The method of claim 4 wherein the nitrogen-containing source is selected from the group consisting of ammonia, hydrazine, monoalkylhydrazine, dialkylhydrazine, nitrogen/hydrogen, ammonia plasma, nitrogen plasma, nitrogen/hydrogen plasma, and mixtures thereof.

9. The method of claim 4 wherein the dielectric film is selected from the group consisting of silicon nitride and silicon carbonitride.

10. A method of forming a dielectric 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 an at least one halogenated organoaminosilane precursor having the following Formula I;

X_mR¹_nH_pSi(NR²R³)_4-m-n-p 

 

I

wherein X is a halide selected from the group consisting of Cl, Br, I;

R¹ is independently selected from a linear or branched C₁ to C₁₀ alkyl group, a C₂ to C₁₂ alkenyl group, a C₂ to C₁₂ alkynyl group, a C₄ to C₁₀ cyclic alkyl, and a C₆ to C₁₀ aryl group;

R² is selected from a linear or branched C₃ to C₁₀ alkyl group, a C₃ to C₁₂ alkenyl group, a C₃ to C₁₂ alkynyl group, a C₄ to C₁₀ cyclic alkyl group, and a C₆ to C₁₀ aryl group;

R³ is selected from a branched C₁ to C₁₀ alkyl, a C₃ to C₁₂ alkenyl group, a C₃ to C₁₂ alkynyl group, a C₄ to C₁₀ cyclic alkyl group, and a C₆ to C₁₀ aryl group;

m is 1 or 2;

n is 0, 1, or 2;

p is 0, 1 or 2; and

the sum of (m+n+p) is less than 4 and wherein R² and R³ are linked to form a ring or R² and R³ are not linked to form a ring;

c. purging the ALD reactor with an inert gas;

d. providing a nitrogen-containing source in the ALD reactor;

e. purging the ALD reactor with an inert gas; and

repeating the steps b through e until a desired thickness of the dielectric film is obtained.

11. The method of claim 10 wherein the at least one halogenated organoaminosilane precursor is selected from the group consisting of 2,6-dimethylpiperidinodichlorosilane, 2,6-dimethylpiperidinochlorosilane, cyclohexylmethylaminochlorosilane, cyclohexylethylaminochlorosilane, and cyclohexyl-iso-propylaminochlorosilane.

12. The method of claim 11 wherein the at least one halogenated organoaminosilane precursor comprises 2,6-dimethylpiperidinodichlorosilane.

13. The method of claim 11 wherein the at least one halogenated organoaminosilane precursor comprises 2,6-dimethylpiperidinochlorosilane.

14. 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, and mixtures thereof.

15. The method of claim 10 wherein the dielectric film is selected from the group consisting of silicon nitride and silicon carbonitride.

16. A method of forming a dielectric film onto at least a surface of a substrate using a plasma enhanced atomic layer deposition (PEALD) process, the method comprising:

a. providing a substrate in an ALD reactor;

b. providing in the ALD reactor an at least one halogenated organoaminosilane precursor having the following Formula I;

X_mR¹_nH_pSi(NR²R³)_4-m-n-p 

 

I

wherein X is a halide selected from the group consisting of Cl, Br, I;

R¹ is independently selected from a linear or branched C₁ to C₁₀ alkyl group, a C₂ to C₁₂ alkenyl group, a C₂ to C₁₂ alkynyl group, a C₄ to C₁₀ cyclic alkyl, and a C₆ to C₁₀ aryl group;

R² is selected from a linear or branched C₁ to C₁₀ alkyl group, a C₃ to C₁₂ alkenyl group, a C₃ to C₁₂ alkynyl group, a C₄ to C₁₀ cyclic alkyl group, and a C₆ to C₁₀ aryl group;

R³ is selected from a branched C₃ to C₁₀ alkyl group, a C₃ to C₁₂ alkenyl group, a C₃ to C₁₂ alkynyl group, a C₄ to C₁₀ cyclic alkyl group, and a C₆ to C₁₀ aryl group;

m is 1 or 2;

n is 0, 1, or 2;

p is 0, 1 or 2; and

the sum of (m+n+p) is less than 4 and wherein R² and R³ are linked to form a ring or R² and R³ are not linked to form a ring;

c. purging the ALD reactor with an inert gas;

d. providing a plasma nitrogen-containing source in the ALD reactor;

e. purging the ALD reactor with an inert gas; and

repeating the steps b through e until a desired thickness of the dielectric film is obtained.

17. The method of claim 16 wherein the at least one halogenated organoaminosilane precursor is selected from the group consisting of 2,6-dimethylpiperidinodichlorosilane, 2,6-dimethylpiperidinochlorosilane, cyclohexylmethylaminochlorosilane, cyclohexylethylaminochlorosilane, and cyclohexyl-iso-propylaminochlorosilane.

18. The method of claim 17 wherein the at least one halogenated organoaminosilane precursor comprises 2,6-dimethylpiperidinodichlorosilane.

19. The method of claim 17 wherein the at least one halogenated organoaminosilane precursor comprises 2,6-dimethylpiperidinochlorosilane.

20. The method of claim 16 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, and mixtures thereof.

21. The method of claim 16 wherein the dielectric film is selected from the group consisting of silicon nitride and silicon carbonitride.

22. A method for forming a silicon oxide film on a substrate comprising:

reacting an oxidizing agent with a precursor comprising a halogenated organoaminosilane represented by the following Formula I;

X_mR¹_nH_pSi(NR²R³)_4-m-n-p 

 

I

wherein X is a halide selected from the group consisting of Cl, Br, I;

R¹ is independently selected from a linear or branched C₁ to C₁₀ alkyl group, a C₂ to C₁₂ alkenyl group, a C₂ to C₁₂ alkynyl group, a C₄ to C₁₀ cyclic alkyl, and a C₆ to C₁₀ aryl group;

R² is selected from a linear or branched C₁ to C₁₀ alkyl group, a C₃ to C₁₂ alkenyl group, a C₃ to C₁₂ alkynyl group, a C₄ to C₁₀ cyclic alkyl group, and a C₆ to C₁₀ aryl group;

R³ is selected from a branched C₃ to C₁₀ alkyl group, a C₃ to C₁₂ alkenyl group, a C₃ to C₁₂ alkynyl group, a C₄ to C₁₀ cyclic alkyl group, and a C₆ to C₁₀ aryl group;

m is 1 or 2;

n is 0, 1, or 2;

p is 0, 1 or 2; and

the sum of (m+n+p) is less than 4 and wherein R² and R³ are linked to form a ring in a vapor deposition process to form the silicon oxide film on the substrate.

23. The method of claim 22 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.

24. The method of claim 22 wherein the at least one halogenated organoaminosilane precursor is selected from the group consisting of 2,6-dimethylpiperidinodichlorosilane, 2,6-dimethylpiperidinochlorosilane, cyclohexylmethylaminochlorosilane, cyclohexylethylaminochlorosilane, and cyclohexyl-iso-propylaminochlorosilane.

25. The method of claim 24 wherein the at least one halogenated organoaminosilane precursor comprises 2,6-dimethylpiperidinodichlorosilane.

26. The method of claim 24 wherein the at least one halogenated organoaminosilane precursor comprises 2,6-dimethylpiperidinochlorosilane.

27. A method for forming a silicon oxide film on a substrate comprising:

forming via vapor deposition of the silicon oxide film on the substrate from a composition comprising at least one organoaminosilane precursor having the following Formula I;

X_mR¹_nH_pSi(NR²R³)_4-m-n-p 

 

I

wherein X is a halide selected from the group consisting of Cl, Br, I;

R¹ is independently selected from a linear or branched C₁ to C₁₀ alkyl group, a C₂ to C₁₂ alkenyl group, a C₂ to C₁₂ alkynyl group, a C₄ to C₁₀ cyclic alkyl, and a C₆ to C₁₀ aryl group;

R² is selected from a linear or branched C₁ to C₁₀ alkyl group, a C₃ to C₁₂ alkenyl group, a C₃ to C₁₂ alkynyl group, a C₄ to C₁₀ cyclic alkyl group, and a C₆ to C₁₀ aryl group;

R³ is selected from a branched C₃ to C₁₀ alkyl group, a C₃ to C₁₂ alkenyl group, a C₃ to C₁₂ alkynyl group, a C₄ to C₁₀ cyclic alkyl group, and a C₆ to C₁₀ aryl group;

m is 1 or 2;

n is 0, 1, or 2;

p is 0, 1 or 2; and

the sum of (m+n+p) is less than 4 and wherein R² and R³ are linked to form a ring or R² and R³ are not linked to form a ring and at least one oxidizing agent,

wherein the vapor deposition is 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.

28. The method of claim 27 wherein the at least one halogenated organoaminosilane precursor is selected from the group consisting of 2,6-dimethylpiperidinodichlorosilane, 2,6-dimethylpiperidinochlorosilane, cyclohexylmethylaminochlorosilane, cyclohexylethylaminochlorosilane, and cyclohexyl-iso-propylaminochlorosilane.

29. The method of claim 28 wherein the at least one halogenated organoaminosilane precursor comprises 2,6-dimethylpiperidinodichlorosilane.

30. The method of claim 27 wherein the at least one halogenated organoaminosilane precursor comprises 2,6-dimethylpiperidinochlorosilane.

31. A method for forming a silicon oxide film on a substrate comprising:

introducing a halogenated organoaminosilane represented by the following Formula I;

X_mR¹_nH_pSi(NR²R³)_4-m-n-p 

 

I

wherein X is a halide selected from the group consisting of Cl, Br, I;

R¹ is independently selected from a linear or branched C₁ to C₁₀ alkyl group, a C₂ to C₁₂ alkenyl group, a C₂ to C₁₂ alkynyl group, a C₄ to C₁₀ cyclic alkyl, and a C₆ to C₁₀ aryl group;

R² is selected from a linear or branched C₁ to C₁₀ alkyl, a C₃ to C₁₂ alkenyl group, a C₃ to C₁₂ alkynyl group, a C₄ to C₁₀ cyclic alkyl group, and a C₆ to C₁₀ aryl group;

R³ is selected from a branched C₃ to C₁₀ alkyl group, a C₃ to C₁₂ alkenyl group, a C₃ to C₁₂ alkynyl group, a C₄ to C₁₀ cyclic alkyl group, and a C₆ to C₁₀ aryl group;

m is 1 or 2;

n is 0, 1, or 2;

p is 0, 1 or 2; and

the sum of (m+n+p) is less than 4 and wherein R² and R³ are linked to form a ring or R² and R³ are not linked to form a ring into a reactor;

introducing at least one oxidizing agent into the reactor wherein the at least one oxidizing agent reacts with the organoaminosilane to provide the silicon oxide film on the substrate.

32. A method for forming a silicon oxide film on a substrate wherein the film comprises a thickness, the method comprising:

a. introducing an at least one halogenated organoaminosilane represented following Formula I;

X_mR¹_nH_pSi(NR²R³)_4-m-n-p 

 

I

wherein X is a halide selected from the group consisting of Cl, Br, I;

R¹ is independently selected from a linear or branched C₁ to C₁₀ alkyl group, a C₂ to C₁₂ alkenyl group, a C₂ to C₁₂ alkynyl group, a C₄ to C₁₀ cyclic alkyl, and a C₆ to C₁₀ aryl group;

R² is selected from a linear or branched C₁ to C₁₀ alkyl, a C₃ to C₁₂ alkenyl group, a C₃ to C₁₂ alkynyl group, a C₄ to C₁₀ cyclic alkyl group, and a C₆ to C₁₀ aryl group;

R³ is selected from a branched C₃ to C₁₀ alkyl group, a C₃ to C₁₂ alkenyl group, a C₃ to C₁₂ alkynyl group, a C₄ to C₁₀ cyclic alkyl group, and a C₆ to C₁₀ aryl group;

m is 1 or 2;

n is 0, 1, or 2;

p is 0, 1 or 2; and

the sum of (m+n+p) is less than 4 and wherein R² and R³ are linked to form a ring into a deposition chamber;

b. chemisorbing the at least one halogenated organoaminosilane precursor onto the substrate;

c. purging away the unreacted at least one halogenated organoaminosilane precursor using a purge gas;

d. providing an oxygen source to the halogenated organoaminosilane precursor onto the heated substrate to react with the sorbed at least one halogenated organoaminosilaneprecursor; and

e. optionally purging away any unreacted oxygen source.

33. The method of claim 32 wherein steps a. through d. and optional step e. are repeated until the thickness of film is established.

34. The method of claim 32 wherein the at least one organoaminosilane precursor is selected from the group consisting of 2,6-dimethylpiperidinodichlorosilane, 2,6-dimethylpiperidinochlorosilane, cyclohexylmethylaminochlorosilane, cyclohexylethylaminochlorosilane, and cyclohexyl-iso-propylaminochlorosilane.

35. The method of claim 34 wherein the at least one halogenated organoaminosilane precursor comprises 2,6-dimethylpiperidinodichlorosilane.

36. The method of claim 34 wherein the at least one halogenated organoaminosilane precursor comprises 2,6-dimethylpiperidinochlorosilane.

37. The method of claim 32 is an atomic layer deposition process.

38. The method of claim 32 is a plasma enhanced cyclic chemical vapor deposition process.

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

the precursor represented by the following Formula I;

X_mR¹_nH_pSi(NR²R³)_4-m-n-p 

 

I

wherein X is a halide selected from the group consisting of Cl, Br, I;

R¹ is selected from a linear or branched C₁ to C₁₀ alkyl group, a C₂ to C₁₂ alkenyl group, a C₂ to C₁₂ alkynyl group, a C₄ to C₁₀ cyclic alkyl, and a C₆ to C₁₀ aryl group;

R² is selected from a linear or branched C₁ to C₁₀ alkyl, a C₂ to C₁₂ alkenyl group, a C₂ to C₁₂ alkynyl group, a C₄ to C₁₀ cyclic alkyl group, and a C₆ to C₁₀ aryl group;

R³ is selected from a branched C₃ to C₁₀ alkyl group, a C₂ to C₁₂ alkenyl group, a C₂ to C₁₂ alkynyl group, a C₄ to C₁₀ cyclic alkyl group, and a C₆ to C₁₀ aryl group;

m is 1 or 2;

n is 0 or 1;

p is 1; and

the sum of (m+n+p) is less than 4 and wherein R² and R³ are linked to form a ring; and

wherein the purity of the precursor is about 98% or greater.

41. The vessel of claim 40 wherein the vessel is comprised of stainless steel.

42. A halogenated organoaminosilane precursor selected from the group consisting of 2,6-dimethylpiperidinochlorosilane, 2,6-dimethylpiperidinodichlorosilane, dicyclohexylaminochlorosilane, cyclohexylmethylaminochlorosilane, cyclohexylethylaminochlorosilane, cyclohexyl-iso-propylaminochlorosilane, N-(chlorosilyl)perhydroquinoline, phenylallylaminochlorosilane, m-tolylmethylaminochlorosilane, N-(chlorosilyl)carbazole, N-(chlorosilyl)-tetrahydroquinoline, N-(chlorosilyl)indole, N-(chlorosilyl)-2-methylindole, N-(chlorosilyl)-3-methylindole, and N-(chlorosilyl)-1,2,3,4-Tetrahydro-2-methylquinoline.

43. A halogenated organoaminosilane precursor having the following formula:

ClH₂Si(NR²R³)

wherein m=1, n=0, p=2, R² is selected from a linear or branched C₃ to C₁₀ alkyl group and a C₆ to C₁₀ aryl group and R³ is a branched C₃ to C₁₀ alkyl group or a C₆ to C₁₀ aryl group and wherein R² and R³ can form a cyclic or alkyl substituted cyclic ring.

44. A halogenated organoaminosilane precursor having the following formula:

R¹ClHSi(NR³R⁴)

wherein m=1, n=1,p=1, R¹ is a C₁ to C₁₀ alkyl group and R² and R³ are a linear or branched C₃ to C₁₀ alkyl group or a C₄ to C₁₀ aryl group and wherein R² and R³ can form a cyclic or an alkyl substituted cyclic ring.

45. A composition for the deposition of a dielectric film comprising:

X_mR¹_nH_pSi(NR²R³)_4-m-n-p 

 

I

wherein X is a halide selected from the group consisting of Cl, Br, I;

R¹ is independently selected from a linear or branched C₁ to C₁₀ alkyl group, a C₂ to C₁₂ alkenyl group, a C₂ to C₁₂ alkynyl group, a C₄ to C₁₀ cyclic alkyl, and a C₆ to C₁₀ aryl group;

R² is selected from a linear or branched C₁ to C₁₀ alkyl, a C₃ to C₁₂ alkenyl group, a C₃ to C₁₂ alkynyl group, a C₄ to C₁₀ cyclic alkyl group, and a C₆ to C₁₀ aryl group;

R³ is selected from a branched C₃ to C₁₀ alkyl group, a C₃ to C₁₂ alkenyl group, a C₃ to C₁₂ alkynyl group, a C₄ to C₁₀ cyclic alkyl group, and a C₆ to C₁₀ aryl group;

m is 1 or 2;

n is 0, 1, or 2;

p is 0, 1 or 2; and

the sum of (m+n+p) is less than 4 and wherein R² and R³ are linked to form a ring or R² and R³ are not linked to form a ring; and

a solvent selected from the group consisting of an ether, a tertiary amine, a nitrile, an alkyl hydrocarbon, an aromatic hydrocarbon, a tertiary amino ether, or mixtures thereof.