Fin-Waist Formation in β-Ga2O3 Fin Arrays Defined by ICP-RIE

Abstract

Abstract Body: Background. β-Ga2O3 has been actively studied for power electronics, RF devices, etc. Among various device topologies, fin-structures are particularly attractive since they enable the RESURF effect, which is key in designing high-field electronics and enhancement-mode transistor operation1,2. Fin-structures have been successfully employed in demonstrating record-setting devices in β-Ga2O33,4. However, we also observed in these devices that a “waist” tends to develop in the middle of a fin thus current flow can be severely pinched by “narrowed waist”. Here, we report what has been learned to date on fin-waist formation during ICP reactive ion etching (RIE). Experiments. The fin/trench arrays were fabricated on single-crystal β-Ga2O3 substrates with 2 orientations: (010) and (001). Hard masks of Pt were patterned by EBL for fin/trench widths less than 1 μm and optical lithography for wider ones. Definitions of dimensional parameters for fin, trench, fin waist and waist narrowing are illustrated in Fig.1. The dry etch parameters were fixed at 350 W ICP power, 20 W RIE power, 5 mTorr, 20C with 35 sccm BCl3 and 5 sccm Ar. Prior to SEM, the etched samples were soaked in HCl and/or HF to remove etch damage/residue. This dry+wet etch condition has rendered consistently rectangular-shaped single fins on (001) β-Ga2O3 with a fin height/width aspect ratio (AR) up to 9 and used in our reported devices3. Fin-waist dependence on the fin orientation and AR of fins/trenches. Arrays of fins were etched on (001) β-Ga2O3 with fin orientation varied. The fin width was varied from 0.15 to 8 μm, the trench width was varied from 1 to 8 μm, and the fin/trench depth was varied from 1.1 to 2 μm, which corresponds to a fin AR of 0.2-9 and a trench AR of 0.5-1.3. Upon examining cross-sections of these fins/trenches under SEM (Figs. 2-4), we discover following trends: 1) all outermost sidewalls of fin arrays and single fins appear to be vertical, while inner sidewalls all show waist; 2) there is no noticeable dependence on fin orientation; 3) waist narrowing increases from 50±10 nm for a trench AR of 0.5 to 200±10 nm for a trench AR up to 1.3, while appears nearly independent of fin width; and 4) wet etch worsens waist narrowing. Fin-waist dependence on the substrate orientation. Similar trench etching experiments were applied to (010) substrates, given their popular use in device development. Example cross-section SEM images are shown in Fig. 5. Though the ranges of fin/trench geometric parameters are slightly different from the ones explored on the (001) β-Ga2O3, the trends observed are largely consistent across different substrate orientations. Discussions. To advance fin-based device performance, it is essential to increase filling factor of fins with respect to entire area occupied by devices on wafer; the preferred trench AR is >1. Under the present ICP-RIE etch condition, waist narrowing is minimal for a low trench AR; but low trench AR implies a low filling factor of current-carrying fins. Waist narrowing is tolerable for fat fins, but the resultant fins can suffer from severe waist narrowing for large-AR trenches and narrow fins (Fig.6) thus detrimental to realizing smooth current flow in devices. Based on our observations, we hypothesize that in trenches with a high AR, a combination of poor sidewall passivation and ion/electron redistribution can result in appreciable lateral etching thus forming waist in fins or bowing of sidewalls. Similar effects were widely observed in other material systems, and improved passivation of sidewalls and neutralization of charges on sidewalls are common tactics to suppress bowing. Ongoing studies are undertaken to curb this undesirable waist narrowing. 1Hu, et al, EDL, 39, 869(2018). 2Li, et al, TED, 67, 3954(2020). 3Li, et al, IEDM, 270(2019). 4Li, et al, EDL, 41, 107(2020). * This work is in part supported by AFOSR Center of Excellence: ACCESS (FA9550-18-1-0529). CNF uses are partly supported by NSF NNCI-2025233.