Significance
Since its first innovation by Iga and colleagues in the late twentieth century, the use of vertical cavity surface emitting lasers (VCSELs) have rapidly grown to largely impact various areas of industrial applications such as the optical communications. Consequently, they have been explored for potential use as heating sources owing to their numerous advantages in terms of scalability, reliability, and cost-effectiveness. This has been escalated by the recent innovations of large-wafer processes. However, the controllability or achievability of these innovations fully depends on the uniformity of each deposition process for the large wafers which has remained a challenge.
To address these challenges, Hongik University scientists: Youngsu Noh and Yoonsuk Kim (PhD students), Professor Seungho Park and Professor Hyoung June Kim together with Dr. Byung-Kuk Kim from Viatron Technologies in Korea studied carefully the applicability of VCSELs as a heating source for thin-film fabrication processes by proposing optimal operating conditions. In particular, they designed a low-pressure chemical vapor deposition (LPCVD) reactor based on VCSELs with a wavelength of 980nm as the heat source. Their main objective was to realize excellent irradiation uniformity, spatial scalability, and power controllability. Their work is published in the International Journal of Heat and Mass Transfer.
Unlike conventional edge-emitting lasers that emit the beam parallel to the surface plane, VCSELs emits the beam perpendicularly from the top surface, which tends to diffuse slightly. These characteristics were effectively explored to evaluate the irradiation uniformity and spatial scalability for the LPCVD process. For the reactor design, commercial codes on three-dimensional simulation models comprising of conduction, convection, and thermal radiation were utilized to predict the optimal placement of the VCSELs modules for uniform irradiation on the wafers.
The authors observed that the experimentally obtained divergence angle fitting the radiative flux increased with a small slope along with the power load rate even though the average value remained nearly constant. When the experimental and numerical temperature distributions were compared to the silicon wafer exposed to high power VCSEL beams, the present simulation model for the LPCVD reactor design was well validated. To further ensure relatively uniform irradiation on the surface, an optimal distance of 450mm between the modules and the wafer surface was obtained. The contribution of the Arrhenius equations was significant in describing the deposition process of poly-crystalline silicon thin film using silane gas species allowing for the simulation of the deposition process under various operation conditions.
It was worth noting that by using the realistic heat flux boundary conditions, the deposition rates in the wafer edge-region were reduced significantly due to severe temperature drop in the region as compared to ideal uniform temperature conditions. The wafer exclusion in the area was minimized by controlling the VCSEL emitters affecting the edge region to slightly increase the emission power. This meant that the wafer exclusion zone could be significantly decreased without the need for additional structures to stabilize the gas outflow and minimize energy losses in the edge-region that is a common problem in practice.
In summary, the research team successfully explored the application of VCSELs to manufacture LPCVD reactor systems. Based on the results, Professor Seungho Park, corresponding author in a statement to Advances in Engineering expressed his confidence that the results of their study will resolve some of the key challenges in chemical vapor deposition processes.
Reference
Noh, Y., Kim, Y., Park, S., Kim, B.-K., & Kim, H.J. (2019). Applications of vertical cavity surface emitting lasers for low-pressure chemical vapor deposition reactors. International Journal of Heat and Mass Transfer, 141, 245-255.