Chanseob Cho

Fabrication of Silicon Pyramid-Nanocolumn Structures with Lowest Reflectance by Reactive Ion Etching Method

We have developed nanosized structures on a silicon surface by the reactive ion etching method. The purpose of this work is to fabricate pyramid-nanocolumn structures for crystalline silicon solar cells using a reactive ion etching (RIE) system with a metal mesh and to simulate surface texturing structures using the PC1D program. The reflectance of the structures was lower than that of only-pyramid or only-nanocolumn structure arrays. We formed micropyramids by general anisotropic saw damage removal (SDR) etching using potassium hydroxide (KOH) solution, as well as nanocolumn structures by RIE with a metal mesh and SF6/O2 gas (35/30 sccm). We studied the fundamentals and morphologies of structures (micropyramids, nanocolumn structures, and pyramid-nanocolumn structures). Owing to anisotropic SDR etching in KOH solution for a single-crystalline silicon wafer, the whole wafer surface is covered with pyramid structures with a size range of 1–5 µm. After RIE texturing, nanocolumn structures with a size range of 20–50 nm were formed on the pyramid structures. Owing to binary surface texturing with pyramid and nanocolumn structures, spectra with weighted average reflectances below 3% in the wavelength range from 350 to 1100 nm were obtained under optimized condition. In particular, the average reflectance was 1.16% in the wavelength range of 400 to 1000 nm without an antireflection coating (ARC). The combination of the pyramid and nanostructures has a very low reflectance. Therefore, ARC is not required in the solar cell process in the case of using pyramid-nanocolumn structures.

Solar Module with a Glass Surface of AG (Anti-Glare) Structure

Currently, solar module is using the two methods such as a glass-filled method or a super-straight method. The common point of these methods is to use glass structure on the front of solar module. However, the reflectance of the solar module is high depending on the height of the incident sunlight due to the flat surface of the module front glass. Purposed to solve these problems, AG (anti-glare) structures were formed on the glass surface. Next is fabrication methods of AG structure. First, uneven structure made by micro blaster equipment was dipped in Hydro-fluidic acid (HF) acid. HF acid process was carried out to remove particles and to make high transmittance. The reflectance and transmittance of the anti-glare glass was compared to those of the bare glass. The reflectance of anti-glare glass decreased approximately 1% compared with bare glass. The transmittance of anti-glare glass was similar to bare glass. According to the sample angle, the difference of the reflectance between bare glass and the anti-glare glass was about 19%. Isc and efficiency value of anti-glare glass on bare solar cell appeared about 3.01 mA and 0.228% difference compared with bare glass. Anti-glare glass on textured solar cell appeared about 9.46 mA and 0.741% difference compared with bare glass. As a result, the role of anti-glare in the substrate is to reduces the loss of sunlight reflected from the surface. In this study, therefore, AG structure on the solar cell was used to improve the efficiency of solar cell.

Fabrication of Probe Beam by Using Joule Heating and Fusing

Abstract In this paper, we developed a beam of MEMS probe card using a BeCu sheet. Silicon wafer thickness of 400 m was fabricated byusing deep reactive ion etching (RIE) process. After forming through silicon via (TSV), the silicon wafer was bonded with BeCu sheet bysoldering process. We made BeCu beam stress-free owing to removing internal stress by using joule heating. BeCu beam was fused byusing joule heating caused by high current. The fabricated BeCu beam measured length of 1.75 mm and width of 0.44 mm, and thicknessof 15 m. We measured fusing current as a function of the cutting planes. Maximum current was 5.98 A at cutting plane of 150 m 2 . Theproposed low-cost and simple fabrication process is applicable for producing MEMS probe beam. Keywords : BeCu, Fusing, Joule heating, Probe card 1234+ Corresponding author: chocs@knu.ac.kr(Received : Dec. 6, 2012, Revised : Jan. 21, 2013, Accepted : Jan. 21, 2013)This is an Open Access article distributed under the terms of the Creative CommonsAttribution Non-Commercial License(http://creativecommons.org/licenses/by-nc/3.0)which permits unrestricted non-commercial use, distribution, andreproduction in any medium, provided the original work is properly cited.

Nano-size structure formation on Si substrate for optical device application

We have found optimized RIE conditions by increasing the distance between the metal mesh and silicon wafer. We have also found that by increasing the reactive ion etching (RIE) process time with an optimized SF6/O2 ratio, pressure, and RF power. It is possible to switch from a random texture to an nm-size pyramid texture on μm-size pyramid texture. This RIE system textured a crystalline wafer surface that formed approximately 10~20 nm structures on 2~3 μm pyramidal silicon with less than 3% of reflectivity.

Design and Fabrication of Microelectromechanical Systems Probe Card with Vertical Trench Guide for Fine Pitch Probing

In this study, we developed a new microelectromechanical systems (MEMS) probe card with a vertically guided and electroplated cantilever in order to overcome pitch limitation, which has a compliant structure and is capable of a 35 µm pad pitch, with through-wafer interconnections for IC testing. We also investigated the reliability of the MEMS probe card using tribology and scaling of the MEMS probe card for fine pitch probing. The measured contact resistance was less than 0.5 Ω at 1.5 gf applied force and the leakage current was approximately 10 pA between one tip and an adjacent tip. In addition, the planarity of tips and the alignment of x- and y-axes were satisfied with a conventional needle probe card performance. Therefore, this probe card is suitable for wafer-level burn-in testing and function testing of memory and RF devices.

Design and Fabrication of Highly Manufacturable Microelectromechanical Systems Test Sockets for Ball Grid Array Integrated Circuit Packages

We have developed two types of microelectromechanical systems (MEMS) test sockets for the ball grid array (BGA) integrated circuit (IC) packages. The fabricated MEMS test sockets have simple structures, easy fabrication processes, low contact forces, and rapid prototyping and cost-effective processes. In the case of the cantilever-array-type test socket, we optimized the length, width, and thickness of the cantilever appropriate for a 121 ball square BGA IC package test. The contact force is 1.3 mN (1 g force ≈ 1 mN) for each cantilever with a length of 425 µm, a width of 150 µm, and a thickness of 10 µm at a deflection of 100 µm. The mesh-type test socket has a different maximum deflection value in accordance with the contact position. The average contact resistance is 0.73 Ω and the maximum signal path resistance is 18 Ω for the two types of MEMS test sockets. The fabricated MEMS test sockets are suitable for an actual BGA IC package test.

Superhydrophobic surface obtained using pyramidal PTFE film fabricated on RIE etched silicon

We have developed a surface texturing process using a polytetrafluoroethylene coating with a pyramidal structure for obtaining superhydrophobic surfaces. In order to investigate the hydrophobic properties of the surface, we measured the contact angle and roughness values. The calculated roughness factor and root mean square roughness ranged from 2.47 to 2.6 and from 0.25 μm to 0.4 μm, respectively. The contact angle of a water droplet on the surface was greater than 150°; moreover, this angle was maintained for over 7 weeks. This observation implies that extremely low wettability is achievable on superhydrophobic surfaces.

Cantilever arrayed blood pressure sensor for arterial applanation tonometry

We developed a cantilever-arrayed blood pressure sensor array fabricated by (111) silicon bulk-micromachining for the noninvasive and continuous measurement of blood pressure. The blood pressure sensor measures the blood pressure based on the change in resistance of the piezoresistor on a 5-μm-thick-arrayed perforated membrane and 20-μm-thick metal pads. The length and width of the unit membrane are 210 and 310 μm, respectively. The width of the insensible zone between adjacent units is only 10 μm. The resistance change over contact force was measured to verify the performance. The good linearity of the result confirmed that the PDMS package transfers the forces appropriately. The measured sensitivity was about 4.5%/N. The maximum measurement range and resolution of the fabricated blood pressure sensor were greater than 900 mmHg and less than 1 mmHg, respectively.

Fabrication of black silicon by using RIE texturing process as metal mesh

The reduction of optical losses in crystalline silicon solar cell by surface texturing is a critical process to improve the efficiency. We have changed the nonocolumn structure on the micrometer pyramidal structure by RIE texturing.

Fabrication of superhydrophobic surface using surface texturing with nano-sized structure and PTFE film

We developed a superhydrophobic surface using surface texturing with nano-sized structure and polytetrafluoroethylene (PTFE) film deposition. The properties of superhydrophobic surface were investigated using water contact angle, root mean square (RMS) roughness, and X-ray photoelectron spectroscopy (XPS). The contact angle of a water droplet was greater than 150°, which means extremely low wettability is achievable on superhydrophobic surfaces. For nano-sized structure, we carried out two step etching process using reactive ion etching (RIE) in a large pyramid substrate by potassium hydroxide (KOH) solution. Metal mesh installed RIE etched silicon substrate using SF6 and O2 gas. PTFE films were deposited by conventional RF magnetron sputtering. This process is applicable for stable superhydrophobic and self-cleaning surfaces due to hierarchical structures formed silicon wafer etched by RIE technique.