Trilayer Electron-beam Lithography and surface preparation for sub-micron Schottky contacts on GaAs heterostructures

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Foreseen operation at sub-THz frequencies of Schottky contacts for diodes and transistor gates on GaAs based heterostructures requires area reduction down to 0.1×1 microns, and wet chemical processes. We report on the compatibility of Trilayer
   Abstract   — Foreseen operation at sub-THz frequencies of Schottky contacts for diodes and transistor gates on GaAs based heterostructures requires area reduction down to 0.1x1 microns, and wet chemical processes. We report on the compatibility of Trilayer Electron-beam Lithography with such wet processes. I.   I  NTRODUCTION   A  ND B ACKGROUND   HE operation at frequencies above 100 GHz of electronic devices like transistors and diodes has been achieved both  by using high electron mobility III-V semiconductor materials or heterostructures and by implementing fabrication techniques which strongly reduce parasitic capacitances  between the device terminals, without increasing series resistances. Schottky (metal-semiconductor) contacts are normally preferred for diodes and transistor gates, since this kind of junction can ensure a lower density of electrically active surface states between the Gate and the GaAs channel, with consequential benefits for the reduction of the junction current leakage and channel electrons modulation control. However a low density of interface traps, that can substantially influence the electrical properties, requires a very high degree of surface cleaning. Schottky contacts with low capacitance and series resistance are fabricated by electron- beam lithography (EBL) on a triple layer of electronic resists with different sensitivity and subsequent metal evaporation, so to obtain e.g. a “T-shaped” profile of the Schottky metal. This approach provides a small footprint for low capacitance and short electron transit-time in the transistor channel. After  performing a recess on the highly doped semiconductor surface by etching the GaAs trough the footprint opening, a low parasitic device resistance is obtained by depositing a metal Gate which has a low resistance thanks to the larger “head”. High electron mobility transistors (HEMT) have been  produced with a so-called “T-gate” for years, with demonstrated cut-off frequencies well above 300 GHz when the Schottky gate footprint was scaled down to few tens of nanometers [1]. Similarly, Schottky diodes on GaAs have also  been fabricated by using the trilayer EBL technology to  provide optimized geometrical features for the anode electrode, thus achieving cut-off frequencies above 2 THz [2]. It appears then crucial for future terahertz electronic devices to further develop trilayer EBL techniques, by understanding the interaction between different process steps. In this paper we report on the fabrication of Schottky contacts focusing on the effect of wet-etching solutions and surface preparation  processes commonly employed for semiconductor surface cleaning, applied before the Schottky metal evaporation trough the resist apertures. Indeed, since the wet-etching  processes became more critical as the size of the opened areas is reduced, and this requirement became more and more critical for the design of Terahertz electronic devices, the improvement of the etching solution wetting the semiconductor surface trough the resist openings is fundamental. II.   F ABRICATION  The main parameter that limits the operation frequency of Schottky diode is the junction capacitance, which is  proportional to the junction area.   The sub-micron junctions needed for terahertz operation have been fabricated by high-resolution EBL, instead of conventional optical lithography. GaAs heterostructures have been employed for the realization of Schottky contacts with a "T-gate" profile by using a triple e-beam resist coating. In a first configuration of the trilayer the high sensitivity resist which ensures a good lift-off consists in a 33-MMA co- polymer layer. For the fabrication of Schottky contacts, after exposure and development, the optimal surface cleaning is obtained using a basic solution of NH 4 OH:H 2 O (1:30), and the following step for the gate recess is accomplished by using citric acid. However, the above described trilayer is not robust enough to sustain the basic solution since the 33-MMA Co- polymer layer starts to dissolve, producing the collapse of the upper resist layer. Due to this findings we implemented a new type of trilayer in which the second resist layer has a lower concentration of MMA. The first layer for the "foot" definition is a 200nm thick PMMA resist 950K molecular weight. The second one is 8.5 MMA co-polymer with thickness of 640nm. The third layer for the “head” definition is a 130nm thick PMMA 50K molecular weight (see Figure 1). PMMA 950K PMMA 50K 8.5-MMA Figure 1. Scheme of resist profile of the “T-gate” after e-beam exposure and development. Trilayer Electron-beam Lithography and Surface Preparation for Sub-micron Schottky Contacts on GaAs Heterostructures D. Dominijanni a,c , R. Casini a , V. Foglietti a , M. Ortolani a , A. Notargiacomo a , C. Lanzieri, M. Peroni, P. Romanini  b  and E. Giovine a   a CNR - Istituto di Fotonica e Nanotecnologie, Rome, 00156 Italy  b  SELEX Sistemi Integrati, Rome, 00156 Italy c Dipartimento di Ingegneria Elettronica, Università Roma TRE, Rome, 00146 Italy T     Each of the three resist layers is deposited on wafer by spin coating and baked at 170°C on hotplate. We made preliminary exposure tests on semi-insulating GaAs wafer to find the right dose. We optimized the development by using a solution of MIBK:IPA with different concentration in order to open a small “foot” dimension and to obtain a suitable undercut on the second layer obtaining the best result for 1:3 mixture. The determination of the correct dose and development is critical to avoid resist residuals in the aperture and to prepare the surface of the GaAs for the subsequent chemical etch. We investigated the resist developed by scanning electron microscopy (SEM) and identified the right set of doses for the different areas, the highest being the dose for the gate “foot”, which was of the order of 800 C/cm 2 . Once defined the ideal set of doses on semi-insulating wafer, in order to transfer the lithographic process on the GaAs heterostructures, fine adjustment of the dose was required to compensate for the different proximity effect between epitaxial and semi-insulating substrate. We focused our attention on the effect of wet-etching on the resist profile and substrate cleaning. To increase the robustness of the resist to the subsequent chemical processes the wafer is post baked in oven for 30 minutes at 110°C. The trilayer based on 8.5 MMA co-polymer sustain well the surface treatments in NH 4 OH:H 2 O, and in citric acid for the recess, as shown in Figure 2. Figure 2. SEM image of resist profile after surface cleaning Only a little variation of the edge of the upper layer was found, however it is no relevant because the PMMA 50K layer is necessary for the definition of the “head” dimension. On the contrary the undercut of the middle layer is perfectly conserved. Because of the small dimension of the opened areas (less than 250nm) the wettability significantly decreases. To recover a good wettability we treated the surfaces with a dip in a solution of IPA:H 2 O (1:10) for few seconds. In addition we worked to improve the etching solution in citric acid. In particular we investigated the role of temperature and observed that at 35°C the edges of the recess were more homogeneous than at room temperature. A good geometry of the recess is essential for a high breakdown voltage of the HEMT. The fabrication of T-gate Schottky contacts was completed with a metal e-beam evaporation of a thick film of Ti/Al and subsequent lift-off. Cross sectioning by focused ion  beam (FIB) and SEM imaging were used to investigate the  profile of the gates. As shown in Figure 3, the “foot” dimension is closed to 200 nm while the “head” dimension is about 650 nm. This guarantees a very low capacitance and series resistance. It is apparent that the metallic film has a good adhesion to the substrate and no voids are present  between “foot” and “head”, demonstrating the efficiency of the optimized lithographic processes together with the compatibility of the trilayer with the chemical cleaning. The inset of Figure 3 shows the SEM image of the resist  profile for air-bridge Schottky diodes before the metal evaporation. The arrow indicates the foot of the “T-gate”. Figure 3. Frontal cross-section of sub-micron Schottky contact. III.   C ONCLUSION  We optimized a resist trilayer including high sensitivity 8.5 MMA co-polymer for the fabrication of “T-gate” using e- beam lithography. The trilayer is compatible with the chemical  processes necessary for surface cleaning and for the gate recess. Schottky air-bridge contacts have been fabricated on GaAs Heterostructures with “foot” length of 200nm. R  EFERENCES   [1]   X. B. Mei et al, “35-nm InP HEMT SMMIC Amplifier with 4.4-dB Gain at 308 GHz,”  IEEE Electron Device Letters , vol. 28, 2007, pp. 470-472. [2]   P. Siegel, R. Smith, M. Gaidis, S. Martin, “2.5-THz GaAs Monolithic Membrane-Diode Mixer”  IEEE Trans. on Microw. Theo. Techn. , 47, 1999, pp. 596-604   
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