Planarization of GaN by photoresist technique using an inductively coupled plasma

• US 8,257,987 B2
• Filed: 02/02/2007
• Issued: 09/04/2012
• Est. Priority Date: 02/02/2006
• Status: Active Grant

• Current Assignee: Trustees of Boston University
• Original Assignee: Trustees of Boston University
• Inventors: Moustakas, Theodore D., Williams, Adrian D.
• Primary Examiner(s): Toledo, Fernando L
• Assistant Examiner(s): NEWTON, VALERIE N
• Application Number:
  US12/223,505
• Publication Number:
  US 20090236693A1
• Time in Patent Office: 2,041 Days
• Field of Search: None
• US Class Current: 438/22
• CPC Class Codes: H01L 21/30621 Vapour phase etching

Abstract

Films of III-nitride for semiconductor device growth are planarized using an etch-back method. The method includes coating a III-nitride surface having surface roughness features in the micron range with a sacrificial planarization material such as an appropriately chose photoresist. The sacrificial planarization material is then etched together with the III-nitride roughness features using dry etch methods such as inductively coupled plasma reactive ion etching. By closely matching the etch rates of the sacrificial planarization material and the III-nitride material, a planarized III-nitride surface is achieved. The etch-back process together with a high temperature annealing process yields a planarize III-nitride surface with surface roughness features reduced to the nm range. Planarized III-nitride, e.g., GaN, substrates and devices containing them are also provided.

Claims

1. A method for planarizing a surface of a III-nitride substrate or film, the surface comprising surface roughness features, the method comprising:

coating the surface with a layer of sacrificial planarization material to cover the surface roughness features;

etching the sacrificial planarization material to expose the surface roughness features; and

simultaneously etching the sacrificial planarization material and the surface roughness features to reduce the height of the surface roughness features.

2. The method of claim 1, wherein the III-nitride substrate or film is selected from GaN, AlN, InN, AlGaN, InGaN, and AlInGaN.

3. The method of claim 2, wherein the III-nitride substrate or film is GaN.

4. The method of claim 2, wherein the III-nitride substrate or film is grown on sapphire, silicon, silicon carbide, zinc oxide, or glass.

5. The method of claim 2, wherein the III-nitride substrate or film was grown by epitaxy on a crystallographic plane of sapphire selected from the C-plane, the A-plane, the M-plane, and the R-plane.

6. The method of claim 2, wherein the GaN substrate or film is grown by a method selected from hydride vapor phase epitaxy, molecular beam epitaxy, metal organic chemical vapor deposition, liquid phase epitaxy, or sublimation.

7. The method of claim 1, wherein

the ratio of sacrificial planarization material etch rate to III-nitride etch rate is in the range of 0.2 to 1.8;

prior to etching a mean thickness of the layer of sacrificial planarization material is less than 2 times an rms roughness of the surface;

the step of coating comprises spin coating, sputtering, floating, evaporation, melting, stamping, or screen printing;

prior to etching the mean thickness of the layer of sacrificial planarization material is at least 100 Å

greater than the maximum height of said surface roughness features;

the method includes performing said coating and etching steps three or more times;

a step of selecting the RF power coupled into the plasma for a desired etch rate is provided;

a step of selecting the stage bias for a desired etch rate is provided;

the step of etching is performed using a stage bias in the range of -100 to -700 volts;

the final planarized surface is essentially devoid of linear surface defects deviating from the final surface plane by more than 1 μ

m; and

the final planarized surface is essentially devoid of linear surface defects deviating from the final surface plane by more than 10 nm.

8. The method of claim 1, wherein the sacrificial planarization material is selected from a photoresist, an epoxy resin, and spin-on glass.

9. The method of claim 1, wherein the III-nitride material is GaN and the sacrificial planarization material is AZ5214.

10. The method of claim 1, wherein

the step of etching is performed using inductively coupled plasma reactive ion etching;

the plasma comprises a gas selected from the group consisting of Cl₂, BCl₃, a mixture of Cl₂ and BCl₃, and a mixture of Cl₂ BCl₃ and Ar; and

the III-nitride material is GaN.

11. The method of claim 1 further comprising the step of annealing the etched III-nitride surface;

and

wherein

annealing is performed by exposing the surface to an HCl solution or by heating in the presence of GaCl, HCl, H₂, and N₂;

a step of determining the presence or absence of defects in said surface by photoluminescence or cathodoluminescence is provided;

a step of detecting surface defects and if warranted performing an additional annealing step is provided;

wherein the final rms roughness of the surface is less than 3 nm.

12. The method of claim 1, wherein the final rms roughness of the surface is less than 3 nm.

13. The method of claim 1, wherein the step of etching comprises the use of a high density plasma.

14. The method of claim 1, wherein the step of etching comprises the use of an electron cyclotron resonance plasma.

15. The method of claim 1, wherein the final planarized surface is essentially devoid of linear surface defects deviating from the final surface plane by more than 10 nm.

16. The method of claim 1, wherein the surface of the III-nitride material is oriented according to a crystallographic plane selected from the C-plane, the A-plane, the M-plane, and the R-plane.