Polycrystalline aluminum nitride material and method of production thereof
Methods of preparing polycrystalline aluminum nitride materials that have high density, high purity, and favorable surface morphology are disclosed. The methods generally comprises pressing aluminum nitride powders to form a slug, sintering the slug to form a sintered, polycrystalline aluminum nitride boule, and optionally shaping the boule and/or polishing at least a portion of the boule to provide a finished substrate. The sintered, polycrystalline aluminum nitride materials beneficially are prepared without the use of any sintering aid or binder, and the formed materials exhibit excellent density, AlN purity, and surface morphology.
1. A method of preparing a polycrystalline aluminum nitride material having defined characteristics, the method comprising:
pressing aluminum nitride powder to form a pressed aluminum nitride slug; and
sintering the pressed aluminum nitride slug by heating to a maximum temperature of at least about 1,500° C. at a heating rate that does not exceed 0.1° C./minute to 10° C./minute in a non-oxidizing atmosphere to form a sintered aluminum nitride boule with a thickness of at least 5 cm and with a diameter or length or width of at least 5 cm;
wherein the sintered polycrystalline aluminum nitride boule has a relative density of at least about 98% and an aluminum nitride purity of at least about 98% by weight.
2. The method of claim 1 , further comprising mechanically polishing at least a portion of the sintered aluminum nitride boule to form a polycrystalline aluminum nitride substrate.
3. The method of claim 2 , wherein the polycrystalline aluminum nitride substrate has an rms surface roughness of less than about 80 nm and a defect density of less than about 1,000 cm 2 .
4. The method of claim 3 , further comprising, prior to said polishing step, performing a wafering step comprising slicing the sintered aluminum nitride boule into one or more wafers of defined thickness that proceed to said polishing step to form the polycrystalline aluminum nitride substrate.
5. The method of claim 4 , wherein said step of mechanically polishing includes grinding the one or more wafers to a thickness that is less than the thickness immediately after said cutting step.
6. The method of claim 3 , wherein said polycrystalline aluminum nitride substrate has an rms surface roughness of less than about 50 nm.
7. The method of claim 3 , wherein said polycrystalline aluminum nitride substrate has an rms surface roughness of less than about 20 nm.
8. The method of claim 3 , wherein said polycrystalline aluminum nitride substrate has a defect density of less than about 500 cm −2 .
9. The method of claim 3 , wherein said polycrystalline aluminum nitride substrate has a defect density of less than about 200 cm −2 .
10. The method of claim 3 , wherein the defect is a surface depression having a depth of at least about 1.0 μm and a width of at least about 1.0 μm.
11. The method of claim 1 , wherein the aluminum nitride powder has a particle size range such that at least about 50% of the powder particles have a size of less than about 10 μm.
12. The method of claim 1 , wherein the aluminum nitride powder has a particle surface area of at least about 1.0 m 2 /g.
13. The method of claim 1 , wherein the aluminum nitride powder has a particle surface area of about 1.0 m 2 /g to about 10.0 m 2 /g.
14. The method of claim 1 , wherein said pressing is carried out by applying a pressure of at least about 20,000 PSI.
15. The method of claim 1 , wherein said pressing is carried out by applying a pressure of about 20,000 PSI to about 40,000 PSI.
16. The method of claim 1 , wherein said pressing is carried out by increasing the pressure at a rate of about 5,000 PSI/minute to achieve a defined maximum pressure.
17. The method of claim 16 , wherein said pressing further comprises reducing from the maximum pressure to atmospheric pressure in a time of about 1 minute to about 2 hours.
18. The method of claim 1 , wherein said pressing is carried out in the absence of any externally applied heat.
19. The method of claim 1 , wherein said sintering step is carried out in a chamber that is evacuated of ambient gases.
20. The method of claim 19 , wherein said sintering step is carried out in an N 2 atmosphere.
21. The method of claim 20 , wherein the N 2 atmosphere is provided at a pressure of up to about 5 atm.
22. The method of claim 20 , wherein N 2 is provided at a rate of about 10 SCCM to about 3,000 SCCM.
23. The method of claim 1 , wherein said sintering step is carried out such that the maximum achieved temperature is about 1,500° C. to about 2,500° C.
24. The method of claim 1 , wherein the pressed aluminum nitride slug is heated at a constant rate to the maximum sintering temperature.
25. The method of claim 1 , wherein the pressed aluminum nitride slug is heated according to a defined heating algorithm wherein two or more different heating rates are used to achieve successively greater temperatures.
26. The method of claim 25 , wherein the heating algorithm provides for successively slower heating of the pressed aluminum nitride slug.
27. The method of claim 25 , wherein the heating algorithm provides for successively faster heating of the pressed aluminum nitride slug.
28. The method of claim 25 , wherein the heating algorithm provides for alternating between slower heating and faster heating of the pressed aluminum nitride slug.
29. The method of claim 25 , wherein the heating algorithm provides for applying a heating rate that is inversely proportional to the size of the boule being prepared, said heating rate being referenced to the preparation of a plurality of boules.
30. The method of claim 1 , wherein the pressed aluminum nitride slug is heated such that at least a first temperature below the maximum sintering temperature is achieved and the slug is soaked at the first temperature for a time of at least about 1 minute prior to being heated to the maximum sintering temperature.
31. The method of claim 30 , wherein the pressed aluminum nitride slug is heated to a first temperature below the maximum sintering temperature, is soaked for a time of at least about 1 minute at the first temperature, is heated to at least a second temperature below the maximum sintering temperature, is soaked for a time of at least about 1 minute at the second temperature, and is heated to the maximum sintering temperature.
32. The method of claim 1 , wherein the pressed aluminum nitride slug is soaked at the maximum temperature for a time of at least about 2 hours.
33. The method of claim 1 , wherein the sintered aluminum nitride boule is cooled to ambient temperature at a rate of about 0.1° C./minute to about 20° C./minute.
34. The method of claim 1 , wherein said polycrystalline aluminum nitride boule has a relative density of at least about 99% and an aluminum nitride purity of at least about 99% by weight.
35. The method of claim 1 , wherein said pressing and said sintering are carried out in the express absence of any added sintering aid or binder.
36. The method of claim 1 , comprising:
pressing aluminum nitride powder to form a pressed aluminum nitride slug;
sintering the pressed aluminum nitride slug material by heating to a maximum temperature of at least about 1,500° C. at a heating rate that does not exceed 0.1° C./minute to 10° C./minute in a non-oxidizing atmosphere to form a sintered aluminum nitride boule with a thickness of at least 5 cm and with a diameter or length or width of at least 5 cm; and
mechanically polishing at least a portion of the sintered aluminum nitride boule to form a polycrystalline aluminum nitride substrate;
wherein the sintered polycrystalline aluminum nitride substrate has a relative density of at least about 98%, an aluminum nitride purity of at least about 98% by weight, an rms surface roughness of less than about 80 nm, and a defect density of less than about 1,000 cm −2 .