IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 12, NO. 5, MAY 2000

507

Semiconductor Microlenses Fabricated by One-Step Wet Etching Yu-Sik Kim, Jaehoon Kim, Joong-Seon Choe, Young-Geun Roh, Heonsu Jeon, and J. C. Woo

Abstract—We fabricated refractive semiconductor microlenses using a diffusion-limited chemical etching technique based on Br2 solution. The simple one-step wet etching process produced highquality microlenses of GaAs and InP, the two most popular compound semiconductor materials used in optoelectronics. A spherical GaAs microlens with a nominal lens diameter of 30 m exhibited a radius of curvature and focal length of 91 and 36 m, respectively. The surface roughness, examined by atomic force microscopy (AFM), was measured to be below 10 Å. This microlens fabrication method should be readily applicable due to the simplicity in processing and the high-quality results.

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Index Terms—Chemical etching, diffusion process, microlens, refraction, semiconductor materials.

I. INTRODUCTION

M

ICROLENSES and their arrays have many application possibilities in photonics in which devices and systems are ever decreasing in size. They are becoming popular for their ability to manipulate light in a miniature scale and to make the overall packaging simple and compact. Thus, it is anticipated that microlens applications in photonics will widely spread in a variety of formats. A good example is dense two-dimensional (2-D) microlens arrays integrated with vertical-cavity surfaceemitting lasers (VCSEL’s) and photodiodes for parallel data communications through free space [1]. Among others, refractive microlenses formed directly onto semiconductor materials draw much attention because they facilitate monolithic integration with other semiconductor optoelectronic devices, allowing a higher level of integration. Also, the high refractive index of semiconductor materials enables a microlens with large numerical aperture, enhancing the light capture efficiency. The methods developed so far for semiconductor microlens fabrication include photoresist reflow followed by dry-etch [2], [3], mass transport after pre-shaping [4], and shadow mask regrowth [5]. However, those methods require multiple process steps (sometimes even at high temperatures) and/or expensive processing equipment, which are not generally compatible with cost-effective commercial production requirements. In this letter, we propose and describe a method to fabricate semiconductor microlenses in which only a simple one-step chemical Manuscript received September 10, 1999; revised December 14, 1999. This work was supported by the Basic Science Research Institute program (1998–1999), Ministry of Education, Korea. Y.-S. Kim, J. Kim, J.-S. Choe, Y.-G. Roh, and H. Jeon are with the Department of Physics and Inter-University Semiconductor Research Center (ISRC), Seoul National University, Seoul 151-742, Korea. J. C. Woo is with the Department of Physics, Seoul National University, Seoul 151-742, Korea. Publisher Item Identifier S 1041-1135(00)03609-0.

etching process is involved. We also present experimental data to demonstrate the high quality of the processed microlenses. It is well known that chemical etchants used in semiconductor processing can be categorized largely into two groups: reaction-rate-limited and diffusion-limited ones [6]. Most semiconductor wet-etching processes are based on the former, mainly because of the etch selectivity between different materials and the flatness of the etched surface. On the other hand, diffusion-limited etching—typically represented by Br2 solution—has been used in special circumstances. In this etching process, the etched profile is heavily dependent on the diffusion dynamics of the etching species, especially around etch-mask boundaries where the differential change in diffusion (and thus in etching rate) is rapid. Previously, diffusion-limited etching was successfully used to form smooth vertical tapers, crucial for low-loss adiabatic optical mode transformation within a waveguide [7] or diode laser [8], [9]. The process we propose here for microlens fabrication also utilizes the nature of the diffusion-limited etching. Fabrication of our circular microlenses began with the deposition of 1000-Å-thick plasma-enhanced chemical vapor deposition (PECVD) SiN film on a semiconductor substrate. Subsequently, circular holes with an appropriate diameter size were formed in the SiN film by standard photolithography and reactive-ion etching (RIE). The patterned sample was then immersed in a diffusion-limited etchant, which in our study was obtained by mixing HBr : H2 O2 : H2 O in the ratio of 2 : 1 : 60. After a desired etch time, the sample was removed from the etch-bath and thoroughly rinsed in deionized water. The SiN etch mask can be subsequently etched off in HF or by RIE if necessary. Fig. 1 schematically illustrates the etch process inside a mask hole. Br2 molecules do not react with the substrate in the masked region, and those molecules must diffuse into the open area—circular holes in our case—for etch. Due to the low mobility of Br2 molecules, however, the probability that the molecules are consumed is higher near the mask boundary than far away from it. Such gradual spatial variation in the etch rate across the etch window (whose distribution is schematically depicted in the upper panel of Fig. 1) forms a spherical lens profile on the semiconductor surface. During the etch period, which depends on the semiconductor material and the desired lens curvature, the sample needs to sit static inside the etch bath for good reproducibility since any disturbance against the natural diffusive motion of the etching species would alter the details of the etching process. Fig. 2 shows scanning electron microscope (SEM) images of the microlenses that were formed on the (001) surfaces of GaAs and InP substrates by the method described above. The diameter

1041–1135/00$10.00 © 2000 IEEE

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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 12, NO. 5, MAY 2000

Fig. 3. An AFM image of a GaAs microlens. The view covers 35 m m.

2 35

Fig. 1. A schematic illustration of the semiconductor microlens fabrication process using Br2 -based diffusion-limited etching. The upper panel shows the distribution of the etch rate across the etch-window opening.

Fig. 4. (a) Surface profile (solid line) for the GaAs microlens along the line through the lens center, together with a spherical fit (dotted line). (b) Amplified profile near the lens center to show the surface roughness. For the purpose of proper scaling, the data are shown after subtraction by the fit.

Fig. 2. SEM images of: (a) GaAs and (b) InP microlenses, fabricated by the proposed process. Facets are formed in the InP microlens, but on the etched sidewalls only.

of the SiN etch holes was kept the same (30 m) while the respective etch duration was 15 and 5 min for GaAs and InP microlenses. As seen in Fig. 2(b), minor faceting appeared for the InP microlens along the [110] direction, which we attribute to the partial reaction-rate-limited etching component in the specific combination of the etchant used and InP. Such faceting may cause slight asymmetry in the lens profile. Nonetheless, apparent faceting was not observed on the surface of the lens itself.

For a detailed quantitative evaluation of lens profile, we employed the AFM technique. Fig. 3 shows the full-scale perspective image of the GaAs microlens, constructed from the measured AFM data. Represented in Fig. 4(a) is the lens profile along the line passing through the center of the lens, which is also reconstructed from the 2-D AFM data. The dotted line is a theoretical fit based on the spherical lens approximation. The radius of curvature of the lens and the focal length estimated from the fit are 91 and 36 m, respectively. A similar analysis for the InP microlens yields a radius of curvature of 159 m and a focal length of 69 m. The obtained lens speed (f=2 for the 30-m GaAs microlens), even though already low enough for the integration with VCSEL’s where the beam divergence angle is small, may be too large for edge-emitting diode laser applications. However, the focal length (thus lens speed) can be easily controlled

KIM et al.: SEMICONDUCTOR MICROLENSES FABRICATED BY ONE-STEP WET ETCHING

by either of two methods described below. First is the spatial control, in which the mask hole diameter is enlarged beyond the diffusion length of the etching species so that the etch-rate slope near the lens center becomes small. The larger the hole size, the flatter the lens profile becomes. There is no practical limitation to aperture size in our method. We have tried up to 120 m in lens diameter, and observed phase error comparable to that of the 30-m case. More useful is the temporal control, where the etch time serves as a convenient lens curvature control parameter. As the etching time gets longer, the etch depth difference between the edge and the center of lens becomes more significant, resulting in the sharper lens profile. Surface roughness is also an important concern in microlenses. Fig. 4(b) is the blowup of the central region of the lens profile shown in Fig. 4(a)—for proper scaling, the data are presented after the subtraction by the fitted background profile. Apart from the gradual modulation around the zero baseline (which reflects a spherical aberration or the discrepancy between the measured data and the fit), the local roughness of the lens surface remains below 610 Å. Such extreme smoothness of the Br2 etched surface had been proven by the high-performance InP laser diodes integrated with vertical mode transformers [8], [9]. Also examined were spatial uniformity of the processed microlenses across the wafer as well as run-to-run reproducibility of our processing. For a 10210 array of a 30-m GaAs microlens with a 250-m period, the standard deviation in focal length was estimated to be 1.8 m, 5% of the mean focal length of 36 m. This deviation is expected to further improve with larger lens arrays due to a reduction of the edge effect. On the other hand, the difference in the mean focal length between two consecutive runs, 3 h apart but in the same etchant batch, was less than 1 m, demonstrating the excellent run-to-run reproducibility of our method. In summary, we applied a one-step diffusion-limited chemical etching technique to GaAs and InP to fabricate high-quality microlenses whose surface roughness was below 610 Å.

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Judging from the various advantages, including the simplicity of the fabrication process and the convenient in situ control of lens curvature with etch time, we believe that the microlens fabrication method proposed and demonstrated in this paper is ideal for photonic device integration and future photonic VLSI device fabrication. ACKNOWLEDGMENT The authors would like to thank S.-D. Lee and Dr. A.-G. Choo at Samsung Advanced Institute of Technology for their technical support with the AFM measurements. They would like to thank Prof. S. M. Lee at Seoul National University for helpful discussions. REFERENCES [1] E. M. Strzelecka, D. A. Louderback, B. J. Thibeault, G. B. Thompson, K. Bertilsson, and L. A. Coldren, “Parallel free-space optical interconnect based on arrays of vertical-cavity lasers and detectors with monolithic microlenses,” Appl. Opt., vol. 37, pp. 2811–2821, 1998. [2] O. Wada, “Ion-beam etching of InP and its application to the fabrication of high radiance InGaAsP/InP light emitting diodes,” J. Electrochem. Soc., vol. 131, pp. 2373–2380, 1984. [3] E. M. Strzelecka, G. D. Robinson, M. G. Peters, F. H. Peters, and L. A. Coldren, “Monolithic integration of vertical-cavity laser diodes with refractive GaAs microlenses,” Electron. Lett., vol. 31, pp. 724–725, 1995. [4] Z. L. Liau, D. E. Mull, C. L. Dennis, and R. C. Williamson, “Large-numerical—Aperture microlens fabrication by one-step etching and masstransport smoothing,” Appl. Phys. Lett., vol. 64, pp. 1484–1486, 1994. [5] G. M. Peake, S. Z. Sun, and S. D. Hersee, “GaAs microlens arrays grown by shadow masked MOVPE,” J. Electron. Mater., vol. 26, pp. 1134–1138, 1997. [6] R. Williams, Modern GaAs Processing Methods. Norwood, MA: Artech House, 1990. [7] T. Brenner and H. Melchior, “Integrated optical modeshape adapters in InGaAsP/InP for efficient fiber-to-waveguide coupling,” IEEE Photon. Technol. Lett., vol. 5, pp. 1053–1056, 1993. [8] H. Jeon, J.-M. Verdiell, M. Ziari, and A. Mathur, “High-power low-divergence semiconductor lasers for GaAs-based 980-nm and InP-based 1550-nm applications,” IEEE J. Select. Topics Quantum Electron., vol. 3, pp. 1344–1350, 1997. [9] H. Jeon, A. Mathur, and M. Ziari, “High power narrow divergence DFB laser diode at 1.55 m,” Electron. Lett., vol. 34, pp. 1313–1315, 1998.

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