Appl Phys A (2012) 106:575–579 DOI 10.1007/s00339-011-6712-6
T-shaped gate AlGaN/GaN HEMTs fabricated by femtosecond laser lithography without ablation Y.D. Du · H.Z. Cao · W. Yan · W.H. Han · Y. Liu · X.Z. Dong · Y.B. Zhang · F. Jin · Z.S. Zhao · F.H. Yang · X.M. Duan
Received: 15 September 2011 / Accepted: 23 November 2011 / Published online: 15 December 2011 © Springer-Verlag 2011
Abstract Femtosecond laser as a maskless lithography technique is able to fabricate structures far smaller than the diffraction limit to a value within sub-micrometer resolution. We present the femtosecond laser lithography without ablation on the positive photoresist is applied in fabricating T-shaped gate AlGaN/GaN HEMT. The feature sizes of femtosecond laser lithography were determined by the incident laser power, the scan speed of the laser focus, the number of scan times, and the substrate materials. T-shaped gate with the smallest gate length 204 nm could be fabricated by dielectric-defined process using femtosecond laser lithography. The fabricated AlGaN/GaN HEMT with 380 nm T-gate exhibits a maximum drain current density of 500 mA/mm and a maximum peak extrinsic transconductance of 173 mS/mm.
Y.D. Du · W. Yan · W.H. Han () · Y.B. Zhang · F.H. Yang Engineering Research Center for Semiconductor Integrated Technology, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China e-mail:
[email protected] Fax: +86-10-82305141 H.Z. Cao · X.Z. Dong · F. Jin · Z.S. Zhao · X.M. Duan Laboratory of Organic NanoPhotonics and key Laboratory of Functional Crystals and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100080, China X.M. Duan e-mail:
[email protected] Y. Liu School of Physics and Engineering, Sun Yat-sen University, Xin-Guang-Xi Rd. 135, Guangzhou 510275, China e-mail:
[email protected]
1 Introduction The AlGaN/GaN high-electron mobility transistors (HEMTs) have shown high power and high frequency performance owing to its combination of high electron velocity, high breakdown electric field and high sheet charge density. During the recent years, the frequency performance of AlGaN/GaN HEMTs has increased steadily due to the optimization of gate resistance, gate capacitance, ohmic resistance, etc. [1–3]. The T-shaped gate is very important for the frequency performance of HEMTs because of the contribution to reducing gate resistance and gate capacitance. Historically, mainly two approaches to form the T-shaped gate structure were considered, namely, the multilayer resist process and dielectric-defined process. In the multiresist process, the engineer relies on the different resist sensitivities to replicate the desired T-shaped gate profile. However, this approach is not highly reproducible and offers less flexibility in achieving the desired T-shaped gate aspect ratio. Regarding this aspect ratio, the dielectric-defined process is superior to the multiresist process because it inherently decouples the formation of the small gate foot from the much larger top portion of the gate [4]. The conventional dielectric-defined T-shaped gate process mainly used the e-beam lithography (EBL) technique with a finely focused electron beam. However, the EBL is a highly cost- and time-consuming technology. Since the dielectric-defined T-shaped gate process is important to the high frequency performance of AlGaN/GaN HEMTs, it is necessary to further investigate a low-cost, high efficiency technique of optical lithography in fabricating a T-shaped gate. Nowadays, sub-100 nm patterns can be achieved in current immersion lithography due to improvements in lens fabrication, mask-making, and light source selection. We are intrigued by the prospects of having available vast numbers of short-wavelength photons and a selection of high
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powers at a variety of wavelengths for selective photochemistry. However, the Raleigh scattering limit imposes a limit on the patterned line-width. Exposure of UV photoresist with femtosecond laser pulse in the near-infrared range is a multiphoton process. Applications of femtosecond lasers in optical lithography take advantage of the dramatic increase in ultraviolet photons over available UV lamps. Nonlinear interaction regimes lead to further reduction of structure sizes in comparison to the pattern of direct laser ablation. The diffraction limit can be exceeded, provided the photochemical processes responsible for molecular scission (positive photoresist) or linking (negative photoresist) have a threshold response to light excitation. Only the central part of the femtosecond laser beam can modify the photoresist and fabricate structures with submicron size. Two-photon excitation of the femtosecond laser locally induced photoresist had successfully achieved a nanometer scale resolution by direct writing [5–7]. Two-photon polymerization on the negative photoresist has been a powerful tool for three-dimensional (3D) photonic devices and micro/nanomachine systems with sub-diffraction-limit features [8–10]. In this letter, we demonstrate an alternative technique of two-photon absorption in which the femtosecond laser exposes the positive photoresist with sub-diffraction-limit features. This technique avoids the damage of laser ablation at high power. We fabricated T-shaped gate AlGaN/GaN HEMTs by the novel maskless lithography technique of the femtosecond laser.
2 Device fabrication The AlGaN/GaN transistor structure was grown on Si substrates by metal–organic chemical vapor deposition (MOCVD). The epilayer consists of a 1700 nm undoped GaN layer and a 25 nm Al0.25 Ga0.75 N barrier layer. Hall measurements of the structure yield an electron sheet density of 2 × 1013 cm−2 and an electron mobility of 1080 cm2 V s at 300 K. This mobility and charge density translate to a 2DEG sheet resistance of 275 /sq. Device mesa was formed using Cl2 /Ar plasma dry etch in an inductively coupled plasma–reactive ion etch (ICPRIE) system. Next, ohmic contacts were formed by rapid multi-step annealing of evaporated Ti/Al/Ni/Au (30 nm/ 120 nm/55 nm/45 nm) at 400°C for 3 min, 700°C for 40 s and 830°C for 30 s in a N2 atmosphere. The ohmic contact resistance was measured to be 0.27 mm by using the transmission line model (TLM) method. In the T-shaped gate process, a 50 nm-thick Si3 N4 layer was deposited by plasma enhanced chemical vapor deposition (PECVD) on top of AlGaN/GaN on silicon substrate.
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A T-shaped gate was defined in the center of the 3 µm drain– source spacing by two-photon femtosecond laser lithography in two steps. The femtosecond laser beam from a femtosecond oscillator (Tsunami, Spectra-Physics: central wavelength set at 780 nm, pulse duration of 80 fs, repetition rate set at 80 MHz) was focused by a 1.45 NA objective lens into the sample of AZ4620. The laser spot was scanned on the focal plane by a mirror set and along the optical axis by a piezo stage, both controlled by a computer. After exposure, the sample was developed in diluted AZ400K to remove the exposed resist. The first exposure defined the foot of the T-shaped gate on the resist using 180 nm-thick AZ4620 which was diluted by PGMEA. Then, the Si3 N4 passivation layer at the foot of the gate was removed by a CHF3 based dry etch in an ICP-RIE system. A slight 10 second “over etch” was performed to insure complete and uniform opening of the Si3 N4 layer. The second overlay exposure defined the head of the gate on the resist using 500 nm-thick AZ4620. Finally, Ni/Au (20 nm/200 nm) T-shaped gate was deposited by e-beam evaporation. The length of the gate foot was in the range of 0.2∼0.5 µm.
3 Experimental results and discussion The feature sizes of femtosecond laser lithography were determined by the incident laser power, the scan speed of laser focus, the number of scan times, and the substrate materials. In order to determine the gate foot sizes precisely, we studied the dependence of the feature size on the factors mentioned above on the 180 nm-thick AZ4620 photoresist. Exposing lines were thinned down when the incident laser power was reduced, as shown in Fig. 1(a), while lines could not be exposed for the incident power below the threshold. Figure 1(b) shows the dependence of the feature size of lithography on the incident laser power for AlGaN/GaN on Si substrate and GaN on sapphire substrate with a single scan at the speed of 10 µm/s. Besides, the feature size depends on the substrate, the line exposure needs smaller incident laser power for AlGaN/GaN on Si substrate than that of GaN on the sapphire substrate. Figure 1(c) shows that the exposure laser power decreased with increasing of the scanning times. Multiple scanning may improve the exposure time so that the lines can be exposed under smaller laser power. Figure 1(d) shows the dependence of the feature size on the scan speed of the laser focus. The high scan speed greatly decreased the exposure time, resulting in very narrow lines. Therefore, the line width was narrowed when the scan speed was increased. Figure 2(a) demonstrates that the transfer line width after etching 50 nm Si3 N4 layer produces a reduction of 10∼100 nm compared to the exposed line width on the photoresist AZ4620. The reduction becomes more obvious when the feature size of exposed line is narrower. The
T-shaped gate AlGaN/GaN HEMTs fabricated by femtosecond laser lithography without ablation
Fig. 1 (a) SEM image of lines fabricated on 180 nm-thick AZ4620 on sapphire substrate with a single scan at a scan speed of 10 µm/s. (b) Feature sizes vs. incident laser power with a single scan for differ-
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ent substrate materials. (c) Feature sizes vs. incident laser power with multiple scans. (d) Feature sizes vs. scan speed
Fig. 2 (a) Feature sizes vs. incident laser power for pattern transfer. (b) SEM image of lines transferred onto Si3 N4 layer
minimum transfer line width on Si3 N4 layer is 201 nm in Fig. 2(b), corresponding to the exposing line width of 297 nm on the photoresist in Fig. 1(a). It means that the residual photoresist at the edge of the lines is not exposed completely, working as a photoresist mask during etching. The gate head was defined by femtosecond lithography of 500 nm-thick AZ4620 photoresist. Figure 3(a) shows the dependence of the feature size on incident laser power on 500 nm-thick AZ4620 photoresist. The feature size in
Fig. 3(a) can be expanded much further by increasing incident laser power or by separately scanning two times with a space of 100 nanometers. Figure 3(b) shows the best T-shaped gate structure in which the 832 nm-long gate head was fabricated on the 204 nm-long gate foot by Ni/Au deposition. The DC performance of AlGaN/GaN HEMTs with a 380 nm T-gate length in Fig. 4(a) was measured using an Agilent B1500 semiconductor parameter analyzer. As shown in
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Fig. 3 (a) Feature sizes of gate head on 500 nm-thick AZ4620 vs. incident laser power. (b) SEM image of a T-shaped gate with a 204 nm gate length
Fig. 4 Plain-view SEM image of fabricated HEMTs (a), output characteristics (b), and transfer characteristics (c) of an AlGaN/GaN HEMT with a 380 nm gate length
Fig. 4(b), the drain-current characteristics of the device exhibit good pinch off behavior. The maximum drain current at VGS = 2 V is 500 mA/mm, the short source-to-drain distance in combination with the low ohmic contact resistance allows a low on-resistance of 4.45 mm and a knee voltage of only 2 V. Figure 4(c) shows the transfer characteristics of the AlGaN/GaN HEMT with a 380 nm gate length. The device exhibits peak transconductance (gm ) of 173 mS/mm at VDS = 7 V. The threshold voltage VTH is defined as the gate bias intercept of the extrapolation of drain current at the point of peak gm . In our device, VTH was determined to
be −1.8 V. The shallower threshold voltage is attributed to the 10 s over-etch on gate foot and incorporates negatively charged fluorine ions into the AlGaN barrier, causing the threshold voltage positive shift [11].
4 Conclusions In summary, an alternative approach based on positive photoresist two-photon femtosecond laser lithography has been demonstrated for fabricating a T-shaped gate. It has the ad-
T-shaped gate AlGaN/GaN HEMTs fabricated by femtosecond laser lithography without ablation
vantage of high resolution, low-cost, and high efficiency. Through the optimization of the incident laser power, the scan speed of the laser focus and the number of scan times, a Ni/Au T-shape gate with the smallest length of 204 nm was fabricated. Finally, we took the novel technique to fabricate 380 nm T-shaped gate AlGaN/GaN HEMTs which exhibit good DC performance, including good pinch off behavior, low on-resistance, and low knee voltage. Acknowledgements The authors gratefully acknowledge Prof. X.D. Wang, Dr. Q. Kan, Mr. S. Chen, Ms. X.N. Xu for discussions and Ms. Y.L. Chen, Mr. C.X. Hu, Ms. L. Jiang, Mr. C. Chang for device processing. This work was sponsored by National Basic Research Program of China (No. 2010CB934104).
References 1. T. Palacios, A. Chakraborty, S. Heikman, S. Keller, S.P. DenBaars, U.K. Mishra, IEEE Electron Device Lett. 27, 13 (2006)
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2. J.W. Chung, W.E. Hoke, E.M. Chumbes, T. Palacios, IEEE Electron Device Lett. 31, 195 (2010) 3. K. Shinohara, A. Corrion, D. Regan, I. Milosavljevic, D. Brown, S. Burnham, P.J. Willadsen, C. Butler, A. Schmitz, D. Wheeler, A. Fung, M. Micovic, in Int. Electron Devices Meet, pp. 672–675 (2010) 4. G.M. Metze, J.F. Bass, T.T. Lee, D. Porter, H.E. Carlson, P.E. Laux, IEEE Microw. Guided Wave Lett. 1, 198 (1991) 5. S. Kawata, H.-B. Sun, T. Tanaka, K. Takada, Nature 412, 697 (2001) 6. S. Juodkazis, V. Mizeikis, K. Seet, M. Miwa, H. Misawa, Nanotechnology 16, 846 (2005) 7. D.-F. Tan, Y. Li, F.-J. Qi, H. Yang, Q.-H. Gong, X.-Z. Dong, X.-M. Duan, Appl. Phys. Lett. 90, 71106 (2007) 8. K. Kaneko, H.-B. Sun, X.-M. Duan, S. Kawata, Appl. Phys. Lett. 83, 2091 (2003) 9. C.-F. Li, X.-Z. Dong, F. Jin, W. Jin, W.-Q. Chen, Z.-S. Zhao, X-M. Duan, Appl. Phys. A, Mater. Sci. Process. 89, 145 (2007) 10. S. Maruo, K. Ikuta, H. Korogi, Appl. Phys. Lett. 82, 133 (2003) 11. Y. Cai, Y.-G. Zhou, K.J. Chen, K.M. Lau, IEEE Electron Device Lett. 26, 435 (2005)
