• Korean Journal of Materials Research
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• v.25 no.1
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• pp.16-20
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• 2015

In this study, in order to improve the efficiency of n-type monocrystalline solar cells with an Alu-cell structure, we investigate the effect of the amount of Al paste in thin n-type monocrystalline wafers with thicknesses of $120{\mu}m$, $130{\mu}m$, $140{\mu}m$. Formation of the Al doped $p^+$ layer and wafer bowing occurred from the formation process of the Al back electrode was analyzed. Changing the amount of Al paste increased the thickness of the Al doped $p^+$ layer, and sheet resistivity decreased; however, wafer bowing increased due to the thermal expansion coefficient between the Al paste and the c-Si wafer. With the application of $5.34mg/cm^2$ of Al paste, wafer bowing in a thickness of $140{\mu}m$ reached a maximum of 2.9 mm and wafer bowing in a thickness of $120{\mu}m$ reached a maximum of 4 mm. The study's results suggest that when considering uniformity and thickness of an Al doped $p^+$ layer, sheet resistivity, and wafer bowing, the appropriate amount of Al paste for formation of the Al back electrode is $4.72mg/cm^2$ in a wafer with a thickness of $120{\mu}m$.

• Proceedings of the Korean Vacuum Society Conference
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• 2012.02a
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• pp.411-412
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• 2012

대부분의 상용 LED는 사파이어기판에 성장된 GaN를 기반으로 사용한다. GaN는 $1,000^{\circ}C$ 이상의 높은 온도에서 성장이 이루어지는데 이 경우 GaN과 사파이어 기판과의 높은 열팽창 계수로 인하여 compressive stress를 받게 된다. 이 compressive stress로 인하여 성장된 GaN wafer에 bowing이 일어나게 되고 이는 기판의 대면적화에 커다란 문제로 작용한다. 이런 문제들을 해결하기 위해 여러 방법이 고안되고 있지만 [1,2], 근본적으로 wafer bowing 문제의 해결은 이루어지고 있지 않다. 한편, 일반적으로 박막을 성장할 때 columnar structure를 가지는 박막이 coalescence되면 박막에 tensile stress가 걸린다는 사실이 알려져 있으며 [3], GaN를 저온에서 성장할 경우 columnar structure를 갖는다는 사실이 보고되었다 [4]. 본 연구에서는 이런columnar structure를 갖는 GaN을 이용하여 wafer bowing 문제가 해결된 GaN 박막 성장을 연구하였다. 본 실험에서는, c-plane 사파이어에 유기금속화학증착법(MOCVD)을 이용하여 nano-columnar structure를 갖는 저온 GaN layer을 성장하였다. 그 후 columnar structure를 유지하면서 $1,040^{\circ}C$까지 annealing한 후 고온에서 flat 한 GaN 박막을 nano-columnar structure GaN layer위에 성장 하였다. 우선 저온 GaN layer가 nano-columnar structure를 갖고, 고온에서도 nano-columnar structure가 유지되는 것을 scanning electron microscopy (SEM)과 transmission electron microcopy (TEM)을 통해 확인하였다. 또한 이런 columnar structure 위에 고온에서 성장시킨 flat한 GaN 박막이 성장된 것을 관찰할 수 있었다. 성장된 GaN박막의 wafer bowing 정도를 측정한 결과, columnar structure를 갖고 있는 고온 GaN 박막이 일반적인 GaN에 비해 확연하게 wafer bowing이 감소된 것을 확인할 수 있었다. Columnar structure가 coalescence가 되면서 생기는 tensile stress가 GaN박막의 성장시 발생하는 compressive stress를 compensation하여 wafer bowing이 줄어든 것으로 보인다. 본 발표에서는 이 구조에 대한 구조 및 stress 효과에 대해서 논의할 예정이다.

• ETRI Journal
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• v.43 no.5
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• pp.909-915
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• 2021

GaAs on Si grown via metalorganic chemical vapor deposition is demonstrated using various Si substrate thicknesses and three types of dislocation filter layers (DFLs). The bowing was used to measure wafer-scale characteristics. The surface morphology and electron channeling contrast imaging (ECCI) were used to analyze the material quality of GaAs films. Only 3-㎛ bowing was observed using the 725-㎛-thick Si substrate. The bowing shows similar levels among the samples with DFLs, indicating that the Si substrate thickness mostly determines the bowing. According to the surface morphology and ECCI results, the compressive strained indium gallium arsenide/GaAs DFLs show an atomically flat surface with a root mean square value of 1.288 nm and minimum threading dislocation density (TDD) value of 2.4×10⁷ cm^-2. For lattice-matched DFLs, the indium gallium phosphide/GaAs DFLs are more effective in reducing the TDD than aluminum gallium arsenide/GaAs DFLs. Finally, we found that the strained DFLs can block propagate TDD effectively. The strained DFLs on the 725-㎛-thick Si substrate can be used for the large-scale integration of GaAs on Si with less bowing and low TDD.

• Journal of the Microelectronics and Packaging Society
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• v.12 no.3 s.36
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• pp.187-196
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• 2005

MEMS devices such as a vibratory gyroscope often suffer from a lower yield rate due to fabrication errors and the external stress. In the decoupled vibratory gyroscope, the main factor that determines the yield rate is the frequency difference between the sensing and driving modes. The gyroscope, fabricated with SOI (Silicon-On-Insulator) wafer and packaged using the anodic bonding, has a large wafer bowing caused by thermal expansion mismatch as well as non-uniform surfaces of the structures caused by the notching effect. These effects result in large distribution in the frequency difference, and thereby a lower yield rate. To improve the yield rate we propose a packaged SiOG (Silicon On Glass) technology. It uses a silicon wafer and two glass wafers to minimize the wafer bowing and a metallic membrane to avoid the notching. In the packaged SiOG gyroscope, the notching effect is eliminated and the warpage of the wafer is greatly reduced. Consequently the frequency difference is more uniformly distributed and its variation is greatly improved. Therefore we can achieve a more robust vibratory MEMS gyroscope with a higher yield rate.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.37 no.9
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• pp.823-828
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• 2013

To reduce the cost per watt of photovoltaic power, it is important to reduce the cell thickness of crystalline silicon solar cells. As the thickness of the silicon layer is reduced, two distinctive thermal expansion rates between the silicon and the aluminum layer induce bowing in a solar cell. With a thinner silicon layer, the bowing distance grows exponentially. Excessive bowing could damage the silicon wafer. In this study, we tried to measure an irregularly curved silicon solar cell more accurately using a 3D image scanner. For the detailed analysis of the three-dimensional bowing shape, a least square fit was applied to the point data from the scanned image. It has been found that the bowing distance and shape distortion increase with a decrease in the thickness of the silicon layer. An Ag strip on top of the silicon layer can reduce the bowing distance.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.28 no.12
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• pp.776-780
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• 2015

In this paper, we have studied the effect of mechanical polishing to Ga-polar face for reducing the wafer bowing and strain in free-standing GaN. After the mechanical polishing to Ga-polar face, the bowing of the free-standing GaN substrate significantly decreased with increasing the size of diamond slurry, and eventually changed the bowing direction from concave to convex. Furthermore, the full width at half maximum (FWHM) of high-resolution X-ray diffraction (HR-XRD) were decreased, especially the FWHM of (1 0 2) reflection for $1.0{\mu}m$ size of diamond slurry was significantly decreased from 630 to 203 arcsec. In the case, we confirmed that the compressive strain in Ga-polar face was fully released by Raman measurement.

• Journal of the Korean Society for Precision Engineering
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• v.22 no.11 s.176
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• pp.165-172
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• 2005

In MEMS (Micro-Electro-Mechanical System), packaging induced stress or stress induced structure deformation becomes increasing concerns since it directly affects the performance of the device. In the decoupled vibratory MEMS gyroscope, the main factor that determines the yield rate is the frequency difference between the sensing and driving modes. The gyroscope, packaged using the anodic bonding at the wafer level and EMC (epoxy molding compound) molding, has a deformation of MEMS structure caused by thermal expansion mismatch. This effect results in large distribution in the frequency difference, and thereby a lower yield rate. To improve the yield rate we propose a packaged SiOG (Silicon On Glass) process technology. It uses a silicon wafer and two glass wafers to minimize the wafer warpage. Thus the warpage of the wafer is greatly reduced and the frequency difference is more uniformly distributed. In addition. in order to increase robustness of the structure against deformation caused by EMC molding, a 'crab-leg' type spring is replaced with a semi-folded spring. The results show that the frequency shift is greatly reduced after applying the semi-folded spring. Therefore we can achieve a more robust vibratory MEMS gyroscope with a higher yield rate.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2008.06a
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• pp.99-99
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• 2008

To grow the large diameter GaN with high structure and optical quality has been obtained by hydride vapor phase epitaxy(HVPE) method. In addition to the nitridation of $Al_2O_3$ substrate, we also developed a "step-growth process" to reduce or to eliminate the bowing of the GaN substrate caused by thermal mismatch during cool down after growth. The as-grown 380um thickness and 75mm diameter GaN layer was separated from the sapphire substrate by laser-induced lift-off process at $600^{\circ}C$. A problem with the free-standing wafer is the typically large bowing of such a wafer, due to the built in the defect concentration near GaN-sapphire interface. A polished G-surface of the GaN substrate were characterized by room temperature Double crystal X-ray diffraction (DCXRD), photoluminescence(PL) measurement, giving rise to the full-width at half maximum(FWHM) of the rocking curve of about 107 arcsec and dislocation density of $6.2\times10^6/cm^2$.

• 한국신재생에너지학회:학술대회논문집
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• 2010.11a
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• pp.43.1-43.1
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• 2010

In the crystalline silicon solar cells, the full area aluminum_back surface field(BSF) is routinely achieved through the screen-printing of aluminum paste and rapid firing. It is widely used in the industrial solar cell because of the simple and cost-effective process to suppress the overall recombination at the back surface. However, it still has limitations such as the relatively higher recombination rate and the low-to-moderate reflectance. In addition, it is difficult to apply it to thinner substrate due to wafer bowing. In the recent years, the dielectric back-passivated cell with local back contacts has been developed and implemented to overcome its disadvantages. Although it is successful to gain a lower value of surface recombination velocity(SRV), the series resistance($R_{series}$) becomes even more important than the conventional solar cell. That is, it is a trade off relationship between the SRV and the $R_{series}$ as a function of the contact size, the contact spacing and the geometry of the opening. Therefore it is essential to find the best compromise between them for the high efficiency solar cell. We have investigated the optimal design for the local back contact by using PC1D simulation.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2006.11a
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• pp.14-14
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• 2006

Homoepitaxial growth of GaN on n-type GaN substrates was carried out by hydride vapor phase epitaxy (HVPE) method. This enables us to reduce or to eliminate the bowing of the GaN substrate caused by thermal mismatch. As a result, the two opposite crystal surfaces have been found to possess low dislocation density. The surface polarity of the homoepitaxially grown GaN was confirmed by both etching of the surface and conversion beam electron diffraction(CBED). The surface morphology and the photoluminescencemeasurement indicated that the surface properties of N-polar face of the homoepitaxlally grown GaN are quite different from the initial N-polar face of the heteroepitaxially grown GaN substrate Also, both surfaces of the GaN substrate were characterized by room temperature Double crystal X-ray diffraction (DCXRD) and photoluminescence measurement.