Chemical Mechanical Polishing of Gallium Nitride

Abstract

Chemical mechanical polishing of gallium nitride with collodial silica has been demonstrated. Removal rates and ultimate surface roughnesses have been found to depend on sample polarity ͑either A or B faced͒ as well as the pH of the polishing fluid and the initial surface morphology. High quality A-faced samples exhibited no polishing action while B-faced material was readily removed. Removal rates at room temperature were found to vary from 0.4 to 1.1 m/h resulting in surface roughnesses as low as 1.1 nm root mean square roughness ͑RMS͒ over 400 m 2 decreasing to 0.4 nm RMS over 25 m 2 . Investigations into the mechanism of polishing reveal both a chemical and mechanical aspect to polishing GaN with colloidal silica.Gallium nitride devices have attracted widespread interest recently as demand increases for highly efficient short wavelength emitters and high power transistor devices. To date, however, the majority of GaN devices developed must be produced using heteroepitaxial techniques on either sapphire or silicon carbide substrates. Large-scale high quality wafers of GaN are not yet readily available even though their usage as homoepitaxial growth substrates is widely considered to be necessary for the next generation of GaN devices. Despite the lack of availability of true bulk substrates for device applications, several techniques exist for generating quasi-bulk substrates including the removal of thick hydride vapor phase epitaxy ͑HVPE͒ GaN layers from their original sapphire growth substrate and high pressure equilibrium growth techniques which produce small single crystal platelets. 1 Both methods require mechanical surface polishing to remove surface features such as hillocks and pits from growth as well as mechanical bowing resulting also from growth and thermal expansion stresses. Polishing with diamond or alumina particles, both harder than GaN, leaves these surfaces ͑and their subsurfaces͒ decorated with scratches and defects. The observations presented here that GaN can be polished damage-free with colloidal silica are the result of an earlier study on processing GaN vertical cavity surface emitting lasers ͑VCSELs͒, 2 devices which are intolerant to the damage produced by abrasive polishing since scattering losses by surfaces and interfaces are proportional to roughness.At a first look, polishing GaN with a much softer colloidal silica material appears to be an exercise in futility, but numerous studies of polishing materials with SiO 2 particles have revealed a curious trend with regard to damage-free removal of material during polishing. Surfaces of silicon, 3,4 GaAs, 5 tungsten, 6 aluminum, interlayer dielectrics ͑ILDs͒, 7 and sapphire 8 each polish at room temperature at high rates activated by the combination of both mechanical polishing and chemical etching. A detailed study of chemical mechanical polishing of sapphire by Gutsche and Moody 8 revealed that the stock removal rate depended on the pH of the stabilized silica suspension, the temperature of the polishing fluid, and the concentratio...

Effect of surfactant and electrolyte on surface modification of c-plane GaN substrate using chemical mechanical planarization (CMP) process

“…Tavernier et al reported CMP of polar GaN face by using Colloidal SiO 2 abrasive particles [13]. Gallium faced samples exhibited no polishing action and N-faced material was readily removed.…”

Dislocation- and crystallographic-dependent photoelectrochemical wet etching of gallium nitride

“…In chemical mechanical polishing ͑CMP͒ studies of GaN, the N-face polishing rate can be as fast as 1.1 m per hour, while no obvious polishing was observed on the as-grown Ga face. 15 Lithographic patterning and MBE regrowth was used to define different lateral regions of Ga-and N-terminated GaN. 10 These regions also exhibited large difference in etch rate in a KOH solution.…”

Etching characteristics of GaN by plasma chemical vaporization machining

“…In addition, 71 min is required for CMP to remove up to the same depth as that removed by RIE from the entire surface of the substrate in 1 min when the machining rate of CMP is 1100 nm/h. [3] Plasma CVM appears to be more suitable for high-speed processing than CMP.…”

“…[1,2] The surface of the bulk GaN substrate is finished by a polishing technique such as chemical mechanical polishing (CMP). [3] GaN is a very hard material; its Poisson ratio, Young's modulus, and Vickers hardness are 0.23, 290, and 12 GPa, respectively. [4,5] Generally, it is difficult to avoid damaging the bulk GaN substrate during surface polishing.…”