Effect of Polishing Parameters on Chemical Mechanical Planarization of C-Plane (0001) Gallium Nitride Surface Using SiO₂and Al₂O₃Abrasives

Abstract

In this study, Chemical Mechanical Planarization (CMP) of (0001) GaN surface, using two different slurries containing colloidal silica and alumina as abrasive nanoparticles, has been reported. Effect of processing parameters, such as concentration of the oxidizer, downward pressure, head rpm, base rpm, pH of the slurries, and type and concentration of abrasive particles, on the material removal rate (MRR) and surface quality (roughness) have been studied in details. The maximum MRR has been found to be ∼39 nm/hr and ∼85 nm/hr for slurries containing SiO 2 and Al 2 O 3 abrasives, respectively, under 38 kPa pressure, 90 rpm base speed (100 rpm for Al 2 O 3 containing slurry), 30 rpm carrier speed, slurry pH 1 (2 for Al 2 O 3 containing slurry), 0.3 M Oxidizer concentration, and 3.75 wt% abrasive particle concentration. RMS surface roughness of 1.3 Å and 0.7 Å, over scanning area of 10 μm × 10 μm and 5 μm × 5 μm, respectively, has been achieved on polished Ga-faced GaN surface for SiO 2 containing slurry using optimized slurry chemistry and processing parameters.Gallium nitride and related iii-v nitride semiconductors are becoming increasingly popular as the candidate material for use in optoand high power and high frequency electronic devices because of their wide bandgaps, high carrier mobility, high saturated electron drift velocity and high breakdown field. 1,2 GaN, in particular, is suitable for variety of electronic and optoelectronic applications because of its unique capabilities of amplifying high-frequency RF signals without distortion, can withstand high temperatures, and emits blue and green lights. However, an important issue in development and commercialization of GaN based power or optoelectronics is the unavailability of suitable device grade (epi-ready) wafer. The ideal choice would be to use homoepitaxial substrate. But, due to difficulties in growth of bulk GaN wafer these substrates are not readily available. The most important challenge for further development of GaN based technology is the need to improve the quality of GaN films by reducing defects caused by the heteroepitaxial growth. GaN, most commonly, is grown heteroepitaxially on sapphire substrate. 1,3 Due to large difference in thermal expansion and lattice mismatch between sapphire and GaN, a large number of crystal defects and rough surface is generated during the growth process, which limits further application of the grown film. 4,5 It is essential to produce an atomically flat, smooth and defect free GaN surface to realize the full potential of this material. Conventional planarization techniques cannot be effectively applied to GaN surfaces because of its extreme mechanical hardness and chemical inertness toward many chemicals. Mechanical polishing using diamond abrasive produces scratches on the surface. Chemical mechanical planarization (CMP) using colloidal silica has been proven successful for preparing defect free GaN surfaces. Numerous reports are available on CMP of several other materials including Al 2 O 3 , 6 SiO 2 , 7 a...

The optimization of chemical mechanical planarization process-parameters of c-plane gallium-nitride using Taguchi method and grey relational analysis

“…Fewer pits observed probably originate from the threading dislocation created during GaN growth and open up or became visible during CMP. The opening up of the dislocations due to the chemical etching effect during CMP is well known as observed in other investigations [28,44,45]. Regarding occasional scratches found after CMP using optimum condition, it is owing to mechanical abrasion by relatively harder Al 2 O 3 particles.…”

“…The CMP process parameters (for KMnO 4 /Al 2 O 3 based slurry) suggested by Taguchi-based grey relational analysis provides a relatively superior combination of MRR and RMS. Past works [21,23,26,28,46] based on traditional experimental techniques-varying one parameter at a time while keeping other parameters constanthave shown that a KMnO 4 /SiO 2 based slurry provides MRR b~40 nm/ h and RMS~b~10 nm. Though the MRR for c-plane GaN for the KMnO 4 /SiO 2 based slurry is significantly lower than that for KMnO 4 / Al 2 O 3 based slurry (from Taguchi-based grey relational analysis), the RMS for KMnO 4 /SiO 2 or H 2 O 2 /SiO 2 based slurry is significantly superior.…”

“…Over the past, the studies on CMP slurries for GaN were focused on various abrasives particles (Al 2 O 3 and SiO 2 ) and chemical oxidizers-KOH, sodium-hypochlorite, H 2 O 2 and KMnO 4 [22][23][24][25][26][27][28][29]. Recently, CMP studies [28] on (0001) GaN with slurry having KMnO 4 as an oxidizer indicate that the MRR for Al 2 O 3 (as abrasive) containing slurry is significantly higher (~double) than that for the slurry having SiO 2 (as abrasive). However KMnO 4 / SiO 2 slurry produced better surface quality in terms of surface roughness (RMS) and relatively defect-free surface compared to the KMnO 4 / Al 2 O 3 slurry.…”

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

“…Highly surface active cationic and/or anionic surfactants are adsorbed on the particle surfaces, which shield the surface charges of the particles due to high ionic strength. In our previous work, and from the literature, KMnO 4 has been proven a good oxidizer for CMP of polar and non-polar GaN surfaces [20,21]. The function of KMnO 4 in GaN CMP is to participate and promote the chemical reaction between the slurry and GaN surface thereby forming a relatively soft oxide (may be Ga 2 O 3 ) layer on the surface, which subsequently is removed by the dissolution and/or mechanical abrasion of polishing.…”

“…Khushnuma et al reported maximum MRR of ∼85 nm/hr on polar GaN wafer for slurry containing 3.75 wt% Al 2 O 3 and 0.3 M oxidizer concentration but no surfactant or electrolyte in the slurry. Material removal rates of 1.14 m/hr and 1.85 m/hr for non-polar (11)(12)(13)(14)(15)(16)(17)(18)(19)(20) and semi-polar (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22) GaN surfaces, respectively, have been achieved using an abrasive free chemical mechanical planarization [20,21]. Studies of surfactant adsorption have typically focused on conditions where there is clear electrostatic driving force for adsorption; i.e., where surfactants have opposite charge to those of metal oxide surfaces [22].…”