Development of III-V Epitaxial Lift-off Processes by Supercritical Fluids Technology

• Main Authors: KUO, HENG, 郭衡
• Other Authors: FENG, JUI-YANG
• Format: Others
• Language: zh-TW
• Published: 2016
• Online Access: http://ndltd.ncl.edu.tw/handle/30964994810584047707

碩士 === 國立高雄大學 === 電機工程學系碩士班 === 104 === Considering the key of high cost for producing high efficiency III-V compound solar cell is mainly due to its expensive substrate. Therefore, developing of reproducible and stable Epitaxial Lift-Off (ELO) technology has been regarded as the key way for cost down issue. Which ELO process is based on selectively attacking a thin sacrificial AlAs layer from GaAs under aqueous hydrofluoric acid (HF) solution (selectivity: >106, as compared to GaAs). Looking into the bottleneck of traditional ELO technology by wet etching method, people found its later etch rate (LER) is mainly dominant by diffusion-limited instead of reaction-rate-limited. In other words, the dominant reason for manipulating etching rate is dependent on the solubility of reaction residual and the exchanging rate of fresh etchant solution in the tunnel. Hence, as sample size is getting larger and the sacrifice layer is going to become narrower (i.e. higher gap-aspect-ratio), it has been expected the etching rate will be more slow down. In order to overcome those bottlenecks caused by traditional wet etching ELO processes, this study was focus on surveying the way for reducing surface tension. We were therefore inspired by supercritical fluids (SCFs) and know it naturally exhibits low viscosity, high diffusivities and zero surface tension – has extremely potential to bring breakthrough in developing ELO technology. Therefore, in the beginning of this study, we aim to establish a highly corrosion-resistance and HF-compatible SCFs etching system for moving into developing SCFs etching technology. Moreover, we launched this study by systematically investigating LER under various SCFs conditions (such as T, P and etchant concentration), accompanying with developing the related thin-film maintained technology. For studying LER by wet etching method, samples with a 500nm thick Au-coated layer (structure: 3µm GaAs thin film - 150nm AlAs sacrificial layer - 350µm GaAs substrate) were patterned to form shapes of square, circle and hexagonal as well as having various size in range of 200µm ~ 1000µm. We hence can evaluate their LER in different concentration of aqueous hydrofluoric acid (HFaq) by determining if the Au-coated GaAs thin film has been lift-off and floating in the etchant. We observed the LER of sample with hexagonal pattern in condition of [HF] = 49% can go to 30 ~ 65µm/min. On the other hand, once the sample area enlarges 400 times (1mm x 1mm → 20mm x 20mm), meanwhile, the ratio of the length of etching tunnel to the thickness of sacrificial layer (gap-aspect-ratio) have one order degree higher (104 → 105). With the same concentration of HF (49%), the LER for 20nm or 100nm AlAs sacrificial layer is 4.6 and 6.2 µm/min, respectively. It also indicates that the speed of ELO processes is mainly affected by diffusion-limit. In the beginning of moving into supercritical fluids etching, we choose anhydrous Hydrogen fluoride-pyridine (HF/Py) as the solute and mix it into the supercritical CO2 (scCO2) as the solvent. By trade off people’s safety and system operation limit, the experiment was conducted under available temperature (T) and pressure (P) in the range of 40oC ~ 60oC and 2000psi ~ 3000psi, respectively. The results show that the LER can be manipulated and enhanced by either of increasing etching temperature or pressure. We therefore set T = 60oC and P = 3000psi for keeping the LER stay in high speed for following studies. Considering mixing low content of pure water into scCO2 can help in taking the residues away from the etching tunnel, in this study, we also confirm that using low concentration aqueous HF(aq) mixed in scCO2 indeed can offer us a higher LER than etching in high concentration of HF/Py. On the other hand, co-solvent effect of mixing acetone (ACE) into HF/scCO2 was also investigated. In case of low [HF] (< 500mM), mixing ACE and HF with the same volume, we found the LER can be little enhanced. However, as increasing [HF] (500 ~ 700 mM), using co-solvent will make the LER have an obvious reducing. Furthermore, in case of [HF] = 713mM, as the volume proportional of ACE to HF is going higher, the LER will be further decreased. In addition, for understanding those etching performance under even higher [HF], by comparing of samples with 150nm AlAs sacrificial layer separately etching in condition of [HF(aq)] = 713mM and [HF(aq)] = 938mM under scCO2, the results shown, in case of [HF(aq)] = 938mM, the LER can be pushed to 111m/min, but damage on the wafer surface was easily accompanying by. Therefore, in this study, we were keeping [HF(aq)] = 713mM (~ 1.24% in wt) to immerse larger samples (size: 20mm x 20mm) in HF/scCO2 until which GaAs thin film can be totally lift-off for realizing their average LER when the needed etching time goes to 0.5 ~ 1 day. The results show that as narrowing the thickness of AlAs from 150nm to 20nm (150, 100, 20nm), the LER will be decreased from 12.8m/min to 7m/min (12.8, 9.3, 7 µm/min). Moreover, comparing to the traditional wet etching ELO method, this study using only 1/40 concentration of HF in scCO2 (i.e. 1.24% V.S. 49%), the LER can be even increased higher than 1.5 times. The niche of introducing SCFs technology into developing “dry” and high efficient ELO processes has been initial demonstrated. Following the ELO process, a 3m GaAs thin film in dimension of 400mm2 has been successfully well-transferred into a flexible PET substrate by using polydimethylsiloxane (PDMS) and NOA61. Overall, in this study, we have built-up a high corrosion-resistance SCFs etching system and introduced it into developing SCFs-related etching technology. We also investigated its etching performances under various pressure and temperature as well as studying its co-solvent effect by acetone. Hopefully, our work in pioneer can become the valuable foundation for developing SCFs etching technology and bring inspired in the field of photonics, Silicon industry, etc.