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GaN Power Devices

Recess Etching Process for AlGaN/GaN-HFET Devices Using In-Situ Monitoring

Presented at CS MANTECH Conference, May 19th – 22nd, 2014, Denver, Colorado, USA

Abstract
SAMCO has developed a dry etching process for AlGaN/GaN-HEMT power devices with precise control of the etching film depth. AlGaN/GaN-HFET devices are “normally on” and require a circuit to turn off the device. To avoid the turning-off of devices, “normally off” devices are preferred for power device applications. One method of making AlGaN/GaN-HFET devices “normally off” is to eliminate the 2D electron gas channel under the gate electrode. This can be achieved by recess etching (for example, etching a 25 nm AlGaN layer with less than 5 nm of the AlGaN film remaining). Precise control of the remaining AlGaN layer thickness is the most crucial factor because the remaining AlGaN layer depth determines Vg-I characteristics of AlGaN/GaN-HFET devices (when V is constant.)
Remaining AlGaN layer thickness was controlled by the etching time while carrying out very slow etching. However, the recess etching requires thickness control on the order of 1 nm and this timed etching method does not allow that. Since the AlGaN etch rate can change due to byproducts in the reaction chamber, the timed etching approach is clearly not effective. This paper introduces SAMCO’s development of a new, precise method for control of the remaining AlGaN layer thickness during recess etching. SAMCO’s new method employs an interferometric film thickness measurement system and in-situ monitoring of the remaining AlGaN thickness.

SiC Power Devices

Optimizing the SiC Plasma Etching Process for Manufacturing Power Devices

Presented at CS MANTECH Conference, May 18th – 21st, 2015, Scottsdale, Arizona, USA

Abstract
In this research, we optimized SiC trench etching using an ICP etching system in order to improve the problems of low SiC etching rate, low selectivity (SiC/SiO2), and micro-trenches. The ICP etching system we used was a SAMCO Model RIE-600iP, which was specifically developed for the purpose of etching SiC. For this SiC trench etching study, we used SF6, O2, and Ar process gases. After verifying that adjusting the process parameters affected the SiC trench etching results, it became clear that we could improve the SiC etching rate by adjusting the bias power, prevent micro-trench generation by adjusting the space between the electrodes (electrode gap), and improve selectivity (SiC/SiO2) by adjusting the process pressure. In doing so, we were able to improve the major problems of SiC trench etching. By adjusting the various parameters and optimizing the SiC trench etch, we were able to achieve an etching rate of 775 nm/min, a selectivity of 13.4 (SiC/SiO2), and a desirable etching shape without micro-trenches.

Through Silicon Via (TSV) Processing

Through Silicon Via (TSV) Process Using DRIE and Cathode Coupled PE-CVD

Presented at The sixth Pacific Rim Meeting on Electrochemical and Solid-State Science (PRiME), Oct 7th – 12th, 2012, Honolullu, Hawaii, USA

Via hole etching and oxide film deposition were carried out using SAMCO’s Deep Reactive Ion Etching (DRIE) and Cathode-coupled  Plasma Enhanced Chemical Vapor Deposition (PECVD). The insulation film showed conformal step coverage and  excellent breakdown voltage.

High Brightness LEDs

Chlorine-Based ICP Etching for Improving the Luminance Efficiency in Nitride LEDs

Presented at CS MANTECH, Apr 23rd – 26th, 2012, Boston, Massachusetts, USA

Abstract
In this research, formation of fine cone-shaped patterns called PSS (Patterned Sapphire Substrate), and reverse-taper etching of GaN sidewalls were carried out using chlorine-based ICP-RIE (Inductively Coupled Plasma – Reactive Ion Etching) in order to improve the external luminance efficiency of GaN-LED (Gallium Nitride – Light Emitting Diodes). Dry etching systems with a special ICP coil called SSTC (Symmetrically Shielded Tornado Coil) [1] are suited for the formation of PSS, and by controlling parameters such as the flow rates of Cl2, BCl3, and Ar, process pressure, ICP RF power, and bias electrode RF power, a cone profile with a curved surface can be formed without forming micro trenches. Also it was confirmed that an etching rate of 100 nm/min, and a selectivity of 1.3 over photo resist (PR) were achieved. Furthermore, reverse-taper etching of GaN was carried out with the same system, and smooth, 70° sidewalls were obtained employing a Ni mask.

Improvement of LED Luminance Efficiency by Sapphire Nano PSS Etching

Presented at CS MANTECH, May 13th – 16th, 2013, New Orleans, Louisiana, USA

Abstract
As a means of improving luminous efficiency of GaN-LEDs (Gallium Nitride-Light Emitting Diodes), pillar or cone shaped structures are periodically generated on sapphire substrates, which is called PSS (Patterned Sapphire Substrates). Using PSS, more light can be extracted (external luminous efficiency is increased) by a scattering effect (using micro-PSS with micrometer-size structures) or by a diffraction effect/photonic crystal effect (using nano-PSS with nanometer size structures). Also it has been reported that nano-PSS is more efficient than micro-PSS. [1]
The most effective way for the formation of nano-PSS is by dry etching using ICP-RIE (Inductively Coupled Plasma-Reactive Ion Etching) and employing photo resist (PR) as a mask. However, there have been challenges related to mask lithography and sapphire etching. In this research, the formation of nano-PSS by dry etching with a PR mask patterned using nano-imprint technology was studied.
First of all, to prevent deformation of the resist mask due to heat from plasma, the mask was UV-cured and hard-baked to increase its heat resistance, and it was confirmed that deformation didn’t take place with processing carried out at 250°C. Secondly, to adjust the height of nano-PSS, etching parameters were studied and optimized. To adjust the height, selectivity against PR mask was crucial, and it was confirmed that selectivity can be controlled by adjusting bias RF Power, process pressure, and CHF3 flow rate.
Lastly, to confirm the EL (electroluminescent) effect of nano-PSS, nano-PSS were fabricated on a sapphire substrates (height 100–750 nm, spacing of 230 nm) and the EL characteristics were compared, after GaN-LED structures were epitaxially grown. The EL intensity using nano-PSS (height 250 nm) was 1.45 times stronger than without PSS formation.

[1] S. Miyoshi, R. Inomoto, N. Okada, K. Tadatomo, T. Nishimiya, M. Hiramoto and S. Motoyama: 11th APCPST & 25th SPSM, October 5, 2012, Kyoto University, Japan

PECVD

Deposition and characterization of silicon carbon nitride films prepared by RF-PECVD with capacitive coupling

Presented at 19th International Symposium on Plasma Chemistry, July 26th – 31st, 2009, Bochum, Germany

Abstract
The goals of this work were to synthesize stoichiometric silicon carbon nitride (Si1.5C1.5N4) films using the RF-PECVD method and to characterize the deposited material. Gas mixtures, as opposed to an organic monomer, were chosen for reactants. Gas mixtures allow for varying the concentration of the elements needed for silicon carbon nitride synthesis and thereby optimizing the composition of the deposited films. It was found that amorphous hydrogenated silicon carbon nitride films having low oxygen contamination and comparable concentrations of silicon and carbon but deficient in nitrogen could be prepared by RF-PECVD from gas mixtures of silane, methane and nitrogen. The methane concentrations in the reactant gas mixture and flow rate or residence time were found to be important va-riables in achieving this objective. The chemical composition, structure and morphology of the films were studied by XPS, HFS, FTIR, XRD and AFM. These data suggested that sili-con bonding in the films was analogous to that in silicon carbide and silicon nitride while carbon was bonded as carbide carbon and nitrogen as nitride nitrogen. The deposited films adhered well to silicon wafers, aluminum and mild steel with minimum pretreatment prior to deposition. The refractive index and density of representative silicon carbon nitride films are also reported.

Empirical evidence for a two phase system in SiCN films prepared by low pressure PE-CVD

Presented at 21st International Symposium on Plasma Chemistry (ISPC 21)  August 4th-9th 2013, Queensland, Australia

Abstract
Silicon carbon nitride films were deposited on silicon wafers at 1,000 °C by RF-PECVD from a gas mixture of silane, methane and nitrogen. The films were analyzed by high resolution XPS, Raman spectroscopy, spectroscopic ellipsometry, profilometry and micro-indentation for hardness and Young’s modulus. The experimental results from this work provide support for the two phase model for silicon carbon nitride that was proposed by others (Liao et al., J Euro Ceram Soc. 2012;32:1275-1281).

Polymer Surface Modification

UV/ozone Surface Modification for Long-term Stable Hydrophilic Surface of Polymer Microfluidic Devices

MRS Advances, 1, pp 743-748. doi:10.1557/adv.2016.167.

Abstract
Faster and more effective surface modification processes of polymer materials by UV/ozone treatment were investigated. The employment of ex-situ generated ozone and/or temperature control contributed to the faster and more effective modification. The UV/ozone treatment showed long-term stable hydrophilic surfaces for 6 months, in contrast to oxygen plasma treatment, which showed hydrophobic recovery. XPS analysis revealed that UV/ozone treatment with ex-situ generated ozone and temperature control added ester (-COOR) on COP sample compared to UV/ozone treatment without the additional ozone and temperature control.

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